[v9.5.x] Run downstream patch check only for grafana/grafana (#83090)

Run downstream patch check only for `grafana/grafana` (#83050)

(cherry picked from commit f683ba8bfc)

Co-authored-by: Fabrizio <135109076+fabrizio-grafana@users.noreply.github.com>

.github/workflows/pr-patch-check.yml

@@ -18,6 +18,7 @@ on:
 jobs:
   trigger_downstream_patch_check:
     uses: grafana/security-patch-actions/.github/workflows/test-patches.yml@main
+    if: github.repository == 'grafana/grafana'
     with:
       src_repo: "${{ github.repository }}"
       src_ref: "${{ github.head_ref }}" # this is the source branch name, Ex: "feature/newthing"