sandbox: __AptDpkgPackageInfo.get_distro_name() returns host distro name

make_sandbox() calls apport.packaging.install_packages() several times. install_packages() tries to install the specified packages, but will fall back to query Launchpad if the package version is not found:

try:
    if ver:
        cache_pkg.candidate = cache_pkg.versions[ver]
except KeyError:
    (lp_url, sha1sum) = self.get_lp_binary_package(self.get_distro_name(),
                                                   pkg, ver, architecture)

The problem is that get_distro_name() returns the host distro name instead of the distro name of the sandbox.