Can't rebuild just subiquity

I can build a snap using `snapcraft snap`. However, when I want to rebuild the `subiquity` part after cleaning (`snapcraft clean subiquity`), I get the following error:

```
Skipping pull probert (already ran)
Skipping build probert (already ran)
Skipping stage probert (already ran)
Skipping prime probert (already ran)
Skipping pull font (already ran)
Skipping build font (already ran)
Skipping stage font (already ran)
Skipping prime font (already ran)
Skipping pull kbdnames (already ran)
Skipping build kbdnames (already ran)
Skipping stage kbdnames (already ran)
Skipping prime kbdnames (already ran)
Skipping pull users-and-groups (already ran)
Skipping build users-and-groups (already ran)
Skipping stage users-and-groups (already ran)
Skipping prime users-and-groups (already ran)
Preparing to pull subiquity
Hit https://download.docker.com/linux/ubuntu bionic InRelease
Hit http://us.archive.ubuntu.com/ubuntu bionic InRelease
Get:1 http://us.archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [83.2 kB]
Get:3 http://us.archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
Fetched 247 kB in 0s (0 B/s)
Pulling subiquity
Cloning into '/home/seth/src/canonicalltd/subiquity/parts/subiquity/src'...
done.
Traceback (most recent call last):
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/__main__.py", line 19, in <module>
    sys.exit(_main())
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/__init__.py", line 246, in main
    return command.main(cmd_args)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/basecommand.py", line 264, in main
    timeout=min(5, options.timeout)) as session:
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/basecommand.py", line 81, in _build_session
    insecure_hosts=options.trusted_hosts,
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/download.py", line 338, in __init__
    self.headers["User-Agent"] = user_agent()
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/download.py", line 101, in user_agent
    zip(["name", "version", "id"], distro.linux_distribution()),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 120, in linux_distribution
    return _distro.linux_distribution(full_distribution_name)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 634, in linux_distribution
    self.version(),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 688, in version
    self.lsb_release_attr('release'),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 836, in lsb_release_attr
    return self._lsb_release_info.get(attribute, '')
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 522, in __get__
    ret = obj.__dict__[self._fname] = self._f(obj)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 933, in _lsb_release_info
    stdout = subprocess.check_output(cmd, stderr=devnull)
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '('lsb_release', '-a')' returned non-zero exit status 1.
Traceback (most recent call last):
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/__main__.py", line 19, in <module>
    sys.exit(_main())
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/__init__.py", line 246, in main
    return command.main(cmd_args)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/basecommand.py", line 264, in main
    timeout=min(5, options.timeout)) as session:
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/basecommand.py", line 81, in _build_session
    insecure_hosts=options.trusted_hosts,
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/download.py", line 338, in __init__
    self.headers["User-Agent"] = user_agent()
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_internal/download.py", line 101, in user_agent
    zip(["name", "version", "id"], distro.linux_distribution()),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 120, in linux_distribution
    return _distro.linux_distribution(full_distribution_name)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 634, in linux_distribution
    self.version(),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 688, in version
    self.lsb_release_attr('release'),
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 836, in lsb_release_attr
    return self._lsb_release_info.get(attribute, '')
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 522, in __get__
    ret = obj.__dict__[self._fname] = self._f(obj)
  File "/home/seth/src/canonicalltd/subiquity/stage/lib/python3.6/site-packages/pip/_vendor/distro.py", line 933, in _lsb_release_info
    stdout = subprocess.check_output(cmd, stderr=devnull)
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/home/seth/src/canonicalltd/subiquity/stage/usr/lib/python3.6/subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '('lsb_release', '-a')' returned non-zero exit status 1.
Failed to run '/home/seth/src/canonicalltd/subiquity/stage/usr/bin/python3 -m pip list': Exited with code 1.
```