Library specs failing on tag pipelines

In !8583 (merged), we moved from git describe to CI_COMMIT_TAG to infer if we are on a tag. But, https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/lib/gitlab/build_iteration.rb still uses git describe, and its tests fail on tag pipelines, as seen from #9264 (closed), #9263 (closed), #9262 (closed), and #9261 (closed).

We need to move everything over to CI_COMMIT_TAG, and fall back to git describe.