Add mamba support to language: conda - #2207

@janjagusch This is missing tests for the *mamba paths. My proposal would be to mock the output of shutil.which and check whether the right conda_exe is used depending on the environment variables set. I wouldn't install mamba / micromamba in CI as it isn't a responsibility of pre-commit to check whether their install commands are drop-ins. If they don't match, this is a bug in the tools themselves.

I went ahead and adjusted this in a few ways and wrote tests:

• if the user opts in via the environment variable don't fall back
• treat any non-empty present variable as set (consistent with the rest of the project)

Thanks @asottile !