Avoid running staleness tests in scheduled CI.

These are designed to only run once daily in "quiet" hours to avoid race conditions.  While isn't very harmful, it does cause our monitoring to show an artificial increase in flakes.

PiperOrigin-RevId: 544883830

.github/workflows/test_runner.yml

@@ -180,6 +180,8 @@ jobs:
     name: Staleness
     needs: [check-tag]
     uses: ./.github/workflows/staleness_check.yml
+    # Staleness tests have scheduled runs during off-hours to avoid race conditions.
+    if: ${{ github.event_name != 'schedule' }}
     with:
       safe-checkout: ${{ needs.check-tag.outputs.checkout-sha }}
     secrets: inherit