ci: add all-green job to pull-request-checks workflow

The repository ruleset requires an 'all-green' status check for PRs
targeting v1.x, but the pull-request-checks workflow was missing this
job. This caused PRs to v1.x to be permanently blocked despite all
individual checks passing.

Add the all-green aggregation job matching the pattern used in main's
CI workflow.

Author: maxisbey

.github/workflows/pull-request-checks.yml

@@ -6,3 +6,12 @@ on:
 jobs:
   checks:
     uses: ./.github/workflows/shared.yml
+
+  all-green:
+    if: always()
+    needs: [checks]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe # v1.2.2
+        with:
+          jobs: ${{ toJSON(needs) }}