CI: add approval validation check for noise changing merge requests

This adds a CI special job that runs only on merge requests that update
the test git ref, indicating that a noise has been changed.  The MR will
show present a report of the changed noises, and will block on MR approval.
Once approval has been given, the check_approval job can be re-run, which
should now pass, allow the MR to proceed.

If the noises change but the reference has not been updated, then the CI
pipeline will fail as usual.