fix(gazelle): runfiles discovery

[aignas]

Pass the environment generated by the runfiles helper so that the
Python launcher can find the runfiles correctly as implemented in
bazelbuild/bazel#14740.

Fixes #1426

[aignas]

@fmeum, could you please take a look? This is an additional change needed on top of your work in #1415.

I have tested it with:

cd examples/bzlmod_build_file_generation
USE_BAZEL_VERSION=last_green bazel run //:gazelle

[fmeum]

Just FYI: With bazel-contrib/bazel-gazelle#1632, the Gazelle wrapper will be setting these variables and this change should only matter for tests at that point.

[rickeylev]

I'm not familiar with Go, so its not clear to me why this works. All it seems to be doing is passing Env() to the subprocess? I don't see Env being modified. Do subprocesses in Go not inherit the current env? I sort of expected to see the PYTHON_RUNFILES envvar being set.

[fmeum]

I'm not familiar with Go, so its not clear to me why this works. All it seems to be doing is passing Env() to the subprocess? I don't see Env being modified. Do subprocesses in Go not inherit the current env? I sort of expected to see the PYTHON_RUNFILES envvar being set.

rfiles.Env() discovers runfiles (potentially by looking next to argv[0]) and then returns the environment variables that subprocesses need to discover runfiles. If no runfiles env variables are set for the outer process, the inner process wouldn't be able to discover runfiles without this line (which has a bug, see my comment).

[rickeylev]

Thanks. LGTM after Fabian's comments are addressed.

[aignas]

I am going to leave this like this due to my observation in #1429 (comment). @rickeylev, @f0rmiga, feel free to merge it during your working hours if you agree to postpone including os.Env() in the std_module.go.