pip install fails for unspecified distros

Pip released version 21.0 and dropped support on python2.7, and python older than 3.6.
This caused 04-install-pip script to fail for some linux distributions (like ubuntu).

Responsible for that is hardcoded execution of:

...
python2 /tmp/get-pip.py $pip_args
...

At the end of file

diskimage-builder/diskimage_builder/elements/pip-and-virtualenv/install.d/pip-and-virtualenv-source-install/04-install-pip

Traceback:

2021-01-25 16:09:47.830 | + python2 /tmp/get-pip.py -U --force-reinstall
2021-01-25 16:09:48.474 | Traceback (most recent call last):
2021-01-25 16:09:48.474 | File "/tmp/get-pip.py", line 24226, in <module>
2021-01-25 16:09:48.479 | main()
2021-01-25 16:09:48.479 | File "/tmp/get-pip.py", line 199, in main
2021-01-25 16:09:48.479 | bootstrap(tmpdir=tmpdir)
2021-01-25 16:09:48.479 | File "/tmp/get-pip.py", line 82, in bootstrap
2021-01-25 16:09:48.479 | from pip._internal.cli.main import main as pip_entry_point
2021-01-25 16:09:48.479 | File "/tmp/tmpRC1lba/pip.zip/pip/_internal/cli/main.py", line 60
2021-01-25 16:09:48.479 | sys.stderr.write(f"ERROR: {exc}")