Previously `ArtifactsController#latest_succeeded` might return 404s even
though `ArtifactsController#download` would successfully return an
artifact. For example, this could happen when:

1. A new commit was merged to `master`.
2. A user attempts to download the artifact for `master` before the
pipeline for `master` finishes, or the pipeline fails altogether.

To fix this problem, we fallback to looking up a successful pipelines by
ref name if one doesn't exist for the SHA.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/216055