ci(pr): fix auto-pr using non-current commit

See https://github.community/t/github-actions-are-severely-limited-on-prs/18179/2

Author: ttshivers

.github/workflows/official-pr.yml

@@ -22,6 +22,7 @@ jobs:
         uses: actions/checkout@v2
         with:
           path: docker-node
+          ref: ${{ github.event.pull_request.head.ref }}
           fetch-depth: 50
 
       - name: Checkout the official-images repo