[9.20] fix: nil: Only run ci-orphaned-anchors on MR events

Now, it is also run in schedules and most annoyingly on push which means
that it is run twice on a push to a branch where a MR exists and `.gitlab-ci.yml` is changed.

This was an oversight in https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/10654

Backport of MR !10668

Merge branch 'backport-stepan/remove-additional-pipeline-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10669