add rule to the ci job (#11511)

Author: sergejparity

scripts/ci/gitlab/pipeline/build.yml

@@ -31,6 +31,8 @@ check-dependent-polkadot:
     COMPANION_OVERRIDES: |
       substrate: polkadot-v*
       polkadot: release-v*
+  rules:
+    - if: $CI_COMMIT_REF_NAME =~ /^[0-9]+$/  #PRs
 
 # TODO: Re-enable after https://github.com/paritytech/pipeline-scripts/issues/44
 #check-dependent-cumulus: