CI: fix git ref in format-check workflow (#37562)

Author: aaronsteers

.github/workflows/format_check.yml

@@ -103,6 +103,7 @@ jobs:
       - name: Checkout code (Unprivileged)
         uses: actions/checkout@v4
         with:
+          ref: ${{ github.event.pull_request.head.ref }}
           fetch-depth: 1
       - name: Set up Python
         uses: actions/setup-python@v5