Allow `tag branch` job to run on merge request pipelines

The docs for CI_COMMIT_BRANCH say:

The commit branch name. Available in branch pipelines, including pipelines for the default branch. Not available in merge request pipelines or tag pipelines.

Since tag branch had if: $CI_COMMIT_BRANCH, it was allowed to run on branch pipelines but not merge request pipelines. This is what caused us to have separate pipelines for branches and MRs, which lead to confusing behavior where a job could pass on the merge request pipeline but fail on the branch pipeline, blocking new images from being pushed.