CI: cancel previous runs for pull requests (#6437)

* CI: cancel previous runs

* Revert "CI: cancel previous runs"

This reverts commit 9faeca2.

* support forked pull requests

Author: chenxsan

.github/workflows/cancel.yml

@@ -0,0 +1,13 @@
+name: Cancel
+on:
+  workflow_run:
+    workflows: ["Ensure PR"]
+    types:
+      - requested
+jobs:
+  cancel:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: styfle/cancel-workflow-action
+      with:
+        workflow_id: ${{ github.event.workflow.id }}