ci: Add back concurrency to cancel in-progress PR commits

Author: WeihanLi

.github/workflows/test.yml

@@ -10,6 +10,11 @@ on:
       - '*'
   pull_request:
 
+# Cancel previous PR branch commits (head_ref is only defined on PRs)
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 env:
   DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true