[9.18] 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-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10670

.gitlab-ci.yml

@@ -591,7 +591,8 @@ ci-orphaned-anchors:
     - bind9-qa/ci-orphaned-anchors/check-orphaned-anchors-ci.py .gitlab-ci.yml
   needs: []
   rules:
-    - changes:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      changes:
         - .gitlab-ci.yml
 
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