Add ability to access env vars that are changed in test-related subprocesses

What’s the problem this feature will solve?

While I was working on #6391 to add a pip cache command, I ran into an issue where environment variables are changed in subprocesses started when a test uses script.pip(...) and similar, but I have no simple way to get this information. See https://github.com/pypa/pip/pull/6391#issuecomment-511151106.

Here’s @chrahunt’s explanation from https://github.com/pypa/pip/pull/6391#issuecomment-511151106:

The basic issue appears to be that changes to APPDATA and LOCALAPPDATA in the environment of the current process do not impact the returned value of ctypes.windll.shell32.SHGetFolderPathW. Only for sub-processes.

One option could be to take the current value of os.environ['LOCALAPPDATA'] and append \pip\Cache to it, duplicating the logic from appdirs.user_cache_dir.

Another option could be to get the cache dir from a sub-process itself, like script.run('python', '-c', 'from pip._internal.locations import USER_CACHE_DIR; print(USER_CACHE_DIR). This seems like more of a hack, but does avoid the duplication.

In either case, it may be good to make it a property of PipTestEnvironment so it’s accessible from the required tests or any others that might need the cache directory.

Describe the solution you’d like

TBD — I’m not sure what approach would be best, and was hoping to get some input on that.

Additional context

PR where this problem came to my attention: https://github.com/pypa/pip/pull/6391

Code where the environment variables are changed: https://github.com/pypa/pip/blob/master/tests/conftest.py#L98-L166

File where PipTestEnvironment, a potential location for tracking this information, is defined: https://github.com/pypa/pip/blob/master/tests/lib/__init__.py

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cc @chrahunt

[pradyunsg]

Let’s delay that – I’m fairly averse to over-engineering things so I don’t think there’s much point in doing this eagerly.

[pradyunsg]

I did that. 😉