Update DCM workflow (#2135)

Author: elliette

.github/workflows/dcm.yml

@@ -4,7 +4,7 @@ name: Dart Code Metrics
 
 on:
   pull_request_target:
-    types: [synchronize, opened, reopened, labeled, unlabeled]
+    types: [opened, reopened, labeled, unlabeled]
     branches:
       - master
 
@@ -30,7 +30,7 @@ jobs:
         name: Checkout repository
         uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab
         with:
-          ref: "refs/pull/${{ github.event.number }}/merge"
+          ref: "${{ github.event.pull_request.sha }}"
       - id: dwds_pub_upgrade
         name: dwds; dart pub upgrade
         run: dart pub upgrade