run-task sometimes fails to check out git repositories when `base_ref` is set

Start at the run-task checkout flow and reproduce the failure with base_ref set to a value such as refs/heads/train-en-ca. Compare checkout behavior with the documented shortened branch-name workaround. Done means run-task successfully checks out the requested repository branch when base_ref includes the refs/heads/ prefix.

This can happen when base_ref is set to a branch like refs/heads/test, we end up with errors like:

[taskcluster 2023-12-04 20:41:36.381Z] === Task Starting ===
[setup 2023-12-04T20:41:38.720Z] run-task started in /
[vcs 2023-12-04T20:41:38.722Z] WARNING: vcs checkout path (/builds/worker/checkouts/src) not in cache or volume; performance will likely suffer
[setup 2023-12-04T20:41:38.722Z] running as worker:worker
[vcs 2023-12-04T20:41:38.722Z] executing ['git', 'clone', 'https://github.com/mozilla/firefox-translations-training', '/builds/worker/checkouts/src']
[vcs 2023-12-04T20:41:38.725Z] Cloning into '/builds/worker/checkouts/src'...
[vcs 2023-12-04T20:41:39.580Z] executing ['git', 'fetch', 'origin', 'refs/heads/train-en-ca']
[vcs 2023-12-04T20:41:39.776Z] From https://github.com/mozilla/firefox-translations-training
[vcs 2023-12-04T20:41:39.776Z]  * branch            train-en-ca -> FETCH_HEAD
[vcs 2023-12-04T20:41:39.783Z] executing ['git', 'checkout', 'refs/heads/train-en-ca']
[vcs 2023-12-04T20:41:39.786Z] error: pathspec 'refs/heads/train-en-ca' did not match any file(s) known to git
[vcs 2023-12-04T20:41:41.789Z] executing ['git', 'checkout', 'refs/heads/train-en-ca']
[vcs 2023-12-04T20:41:41.792Z] error: pathspec 'refs/heads/train-en-ca' did not match any file(s) known to git
[vcs 2023-12-04T20:41:45.796Z] executing ['git', 'checkout', 'refs/heads/train-en-ca']
[vcs 2023-12-04T20:41:45.800Z] error: pathspec 'refs/heads/train-en-ca' did not match any file(s) known to git

The known workaround at the moment is to shorten the base_ref to only include the branch name, eg: with something like:

             short_base_ref:
                  $if: 'base_ref[:11] == "refs/heads/"'
                  then: {$eval: 'base_ref[11:]'}
                  else: ${base_ref}