`session.install` fails on MSYS2 virtualenv

[davidhewitt]

Current Behavior

Install nox using MSYS2 Python. (Use setup-msys2 action to create an environment on Github actions.)

session.install(<anything>) will fail, with errors such as the following:

nox > Error: python is not installed into the virtualenv, it is located at D:/a/_temp/msys64/mingw64/bin/python.EXE. Pass external=True into run() to explicitly allow this.

I tried to use --no-error-on-external-run flag to workaround this, but it looks like the implementation of session.install hard forces external="error" so this makes it a hard blocker.

Expected Behavior

nox is able to install required dependencies into the virtualenv.

Steps To Reproduce

See https://github.com/davidhewitt/msys2-nox-repro for a minimal repro to set up an environment in Github actions which reproduces this failure.

Failing run is e.g. https://github.com/davidhewitt/msys2-nox-repro/runs/7296705086?check_suite_focus=true

Environment

- OS: Windows 10
- Python: 3.10 (MinGW / MSYS2)
- Nox: 2022.1.7

Anything else?

No response