Follow-up from "ci: Tag QA image with the commit SHA instead of branch name"

The following discussion from !66390 (merged) should be addressed:

• @godfat-gitlab started a discussion: (+1 comment)

  @rymai Thank you! Looks good. I wonder if we can reuse the QA_IMAGE variable somehow? For example:

  variables:
    QA_IMAGE_SHA: "${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA:-$CI_COMMIT_SHA}"
    QA_IMAGE: "${CI_REGISTRY}/${CI_PROJECT_PATH}/gitlab-ee-qa:${QA_IMAGE_SHA}"

  If we cannot use conditions in CI variables clause, maybe some of the jobs can set that in before_script. Would you like to try that? I think we can go with the current changes, too.