Fix workflow concurrency to prevent PR builds from cancelling main builds

Previously, PR builds and main branch push builds shared the same
concurrency group because pull_request_target events use the base
branch ref. This caused PR builds to cancel ongoing main builds.

Now PR builds use a unique concurrency group (pr-<number>) while
push builds continue to use the branch ref, preventing interference.

Co-Authored-By: Claude <[email protected]>

Author: dahlia

.github/workflows/build.yaml

@@ -2,7 +2,7 @@ name: build
 on: [push, pull_request_target]
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request_target' && format('pr-{0}', github.event.pull_request.number) || github.ref }}
   cancel-in-progress: true
 
 jobs: