cmd/go/internal/vcsweb/vcstest: skip bzr test if deps not found

cmd/go/internal/vcsweb/vcstest: skip bzr test if deps not found

The linux-arm64 trybots are consistently failing with

    vcstest_test.go:155: 2025/01/30 21:50:41 hello.txt:
        > handle bzr
        > env BZR_EMAIL='Russ Cox <rsc@google.com>'
        > env EMAIL='Russ Cox <rsc@google.com>'
        > bzr init-repo .
        [stderr]
        brz: ERROR: Couldn't import breezy and dependencies.
        Please check the directory containing breezy is on your PYTHONPATH.

        Error: PyErr { type: <class 'ModuleNotFoundError'>, value: ModuleNotFoundError("No module named 'breezy'"), traceback: None }

    vcstest_test.go:161: hello.txt:6: bzr init-repo .: exit status 1

This seems to be a problem with the builder.

For now, skip the test if we see that error message, just as we already
skip the test if the bzr executable is not found.

For #71504

Change-Id: If8b6d4dea02dc16198ba6067595dff3340a81299
Reviewed-on: https://go-review.googlesource.com/c/go/+/646635
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>