Limit CI concurrency to one per PR

In current workflow, when newer commit comes, old pipeline won't get
canceled, which wastes resources and cause unnecessary CI failure
emails. This PR limits concurrency to one per PR. when newer
commit comes, stale CI jobs will be automatically cancelled.

See: https://stackoverflow.com/a/72408109/8711684

.github/workflows/build.yml

@@ -4,7 +4,11 @@ on:
     types: [opened, synchronize, reopened, ready_for_review]
     branches:    
       - main
-
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }} # from https://stackoverflow.com/a/72408109/8711684
+  cancel-in-progress: true
+
 env:
   BUILD_TYPE: "debug"
   FTS_MODE: "internal_fts"