[CI][Go] Run benchmark steps only on the main branch

[kou]

Describe the enhancement requested

Is there any reason we are raising an AssertionError here?

Yes, this code is not supposed to execute in GHA for a non-default branch:

arrow/ci/scripts/go_bench_adapt.py

Line 46 in e6221c4

"pr_number": None, # implying default branch

else:
    # Assume that the environment is not GitHub Actions CI. Error out if that
    # assumption seems to be wrong.
    assert os.getenv("GITHUB_ACTIONS") is None

    # This is probably a local dev environment, for testing. In this case, it
    # does usually not make sense to provide commit information (not a
    # controlled CI environment). Explicitly keep `github_commit_info=None` to
    # reflect that (to not send commit information).

In that sense I think the failure is expected and does / should not block merge. If the redness is a problem then we can also sys.exit(0) instead. Although that might result in mean false-positive green signal in the future.

Originally posted by @jgehrcke in #36195 (comment)

---

Ok, I think the problem is that the PR from dependabot is coming from a branch on the apache/arrow repository instead of a fork hence those are executed which are not if they are from a fork branch due to:
if: success() && github.event_name == 'push' && github.repository == 'apache/arrow'
Probably we should not error and just do a sys.exit(0) but now I understand why this is failing here.

Originally posted by @raulcd in #36195 (comment)

Component(s)

Continuous Integration, Go