[bazel] Add PYTHONUSERBASE to list of non-hermetic env vars

[jwnrt]

We use python modules from outside of Bazel's "hermetic seal".

Without allowing this variable through we always assume they're in the default location.

For environments which set PYTHONUSERBASE, this leads to confusing behaviour where modules are installed and work outside of Bazel, but not inside.

[jwnrt]

@HU90m am I misremembering that you were working on bringing the python modules into the Bazel environment? If that work's ongoing, it can replace this.

[HU90m]

Haha LGTM

Does it need a comment? I guess it's pretty self explanatory so people can look it up.

[HU90m]

@HU90m am I misremembering that you were working on bringing the python modules into the Bazel environment? If that work's ongoing, it can replace this.

I was considering trying to make it a little more hermetic, but there are bigger fish to fry so I doubt I'll ever get round to it.

[jwnrt]

Does it need a comment?

Sure, added a comment

[jwnrt]

CI failures were an upstream problem. Rebased on the fix and it's passing now.

[a-will]

The "real fix" would probably be to eliminate system Python use in the build. I'm not sure how big of a problem that is, though. Some work needs to be done with our fusesoc generators so they use the bazel-provided Python, for example.

Edit: ...I tried just passing the interpreter around by modifying the PATH in a shim py_binary for fusesoc, and while that handled calling the correct interpreter for the generators, that Python environment didn't come with many of the modules available from @ot_python_deps 😅 . primgen would immediately fail to load mako as a result.

Stuffing in all_requirements into that py_binary shim's deps then did the trick. Considering this represents much more than fusesoc's direct dependencies, it still doesn't feel quite right, but this mechanism is probably closer to where we'd want to go.

[jwnrt]

@a-will do you have any objection to landing this as a stop gap between having python managed by Bazel?

[a-will]

@jwnrt The work for fusesoc was done in #19476. Are there any remaining uses of Python in bazel where PYTHONUSERBASE would be needed?

[jwnrt]

Nope, I've been going fine for a while without this env var propagated. Thanks @a-will.