Add an extra job that checks if nothing changed outside of `qa/`

The following discussion from gitlab-ce!16096 should be addressed:

• @godfat commented on a discussion: (+3 comments)

  @grzesiek Does it make sense if we have a special job for qa- or -qa which would check if nothing outside qa/ is changed?

  And follow up is https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16179