Add support for @ENV as an exe preference

[JamesWrigley]

This will force PythonCall to read the executable from the environment variable. Quoting the new docs:

The advantage of setting @ENV as a preference is
that CondaPkg.jl will not be loaded, which saves on load time, while you still
keep the ability to change Python environments based on environment
variables. This can be helpful when using shared Julia environments on a
cluster so that users can easily switch Python environments without modifying
the depot preferences.

When implementing this I also noticed that in embedded mode we set CTX.exe_path by the preference, which was wrong before (coincidentally fixed just now by #814 😛) and seems a bit fragile, so in 777fac8 I changed it to read it from sys.executable instead. Also wrapped a few functions from the stable ABI to help.

[JamesWrigley]

AFAICT the failures are unrelated 👀

[JamesWrigley]

Debugged the CondaPkg test failures to an invalid cached build of Conda.jl being used, fixed in 44aa949 by always building PyCall. The benchmark job failure seems to be because this PR is from my fork and doesn't have privs to comment.