Consider not rebuilding QA images since they're built in GitLab's pipelines already

In the GitLab pipelines, we already build the QA image (for the Merge Result): https://gitlab.com/gitlab-org/gitlab/-/blob/7bec95a990a0da44bc5c778ca78cf090b77f589b/.gitlab/ci/review.gitlab-ci.yml#L1-10

It looks like omnibus-gitlab's pipelines rebuild a QA image (potentially for the non Merge Result commit, though, due to https://gitlab.com/gitlab-org/gitlab/-/blob/7bec95a990a0da44bc5c778ca78cf090b77f589b/scripts/trigger-build#L141) so this may duplicate the work for no good reason.

The fact that in GitLab's pipelines the image is build for the Merge Result and not in omnibus-gitlab's pipelines is actually one good reason to use the image built in GitLab's pipeline since it would mean the image is "closer" to master.

One thing to solve, though is that the image is tagged as ${CI_REGISTRY}/${CI_PROJECT_PATH}/gitlab-ee-qa:${CI_COMMIT_REF_SLUG} (https://gitlab.com/gitlab-org/gitlab/-/blob/7bec95a990a0da44bc5c778ca78cf090b77f589b/.gitlab/ci/review.gitlab-ci.yml#L8), but the omnibus-gitlab is tagged with the commit SHA (e.g. https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/587107397), as well as the QA image (e.g. https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/587107399), and the QA image in gitlab-qa-mirror is inferred from it (e.g. https://gitlab.com/gitlab-org/gitlab-qa-mirror/-/jobs/587211159).