Correctly populate the system python path

[frostming]

Thank you for contributing to Pipenv!

The issue

Close #4311, Close #4315

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix, .feature, .behavior, .doc. .vendor. or .trivial (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.

[jandd]

I think the python binary at least on Linux systems should be python3 if Python 3.x is used. If I just set the environment variable PIPENV_USE_SYSTEM=1 on a Python 3 only system with the current pipenv I still get an error because it tries to use /usr/bin/python which does not exist:

Installing dependencies from Pipfile.lock (63b55f)…
Failed to load paths: /bin/sh: /usr/bin/python: not found

I just added export PIPENV_USE_SYSTEM=1 to the Dockerfile mentioned in #4315 to test this.

[frostming]

@jandd thanks for catching this, can you test with the latest changes?

[jandd]

@frostming the applied patch set makes pipenv install --system --deploy work in the Alpine 3.12/Python 3.8 container

[techalchemy]

makes sense to me... I was considering changing this behavior before the last release but I couldn't convince myself it was a good idea. Thanks for tackling it.