Fix the DCM workflow (#2147)

Author: elliette

.github/workflows/dcm.yml

@@ -30,7 +30,7 @@ jobs:
         name: Checkout repository
         uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
         with:
-          ref: "${{ github.event.pull_request.sha }}"
+          ref: "${{ github.event.pull_request.head.sha }}"
       - id: dwds_pub_upgrade
         name: dwds; dart pub upgrade
         run: dart pub upgrade