The `gitlab:assets:compile` job isn't run for the QA branches, thus
there's no Docker image correspinding these branches in the registry.

By overriding `CI_COMMIT_REF_SLUG` to `master` for QA branches, the
`fetch-assets` job in the `omnibus-gitlab` pipeline will pull the
`master` assets Docker image.
Signed-off-by: Rémy Coutable <remy@rymai.me>