Run DCM workflow against PR branch (#2131)

Author: elliette

.github/workflows/dcm.yml

@@ -29,6 +29,8 @@ jobs:
       - id: checkout
         name: Checkout repository
         uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
+        with:
+          ref: "refs/pull/${{ github.event.number }}/merge"
       - id: dwds_pub_upgrade
         name: dwds; dart pub upgrade
         run: dart pub upgrade