Override CI_COMMIT_REF_SLUG for QA branches

What does this MR do?

This solves an edge-case when QA branches wouldn't run the gitlab:assets:compile job but the omnibus-gitlab pipeline (triggered by the package-and-qa job) would try to retrieve the assets Docker image using GITLAB_REF_SLUG, which defaults to CI_COMMIT_REF_SLUG in CE/EE.

By overriding CI_COMMIT_REF_SLUG to master when the branch is a QA branch, the fetch-assets in the omnibus-gitlab pipeline will retrieve the master assets Docker image.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.