CI: cancel stale workflows.

Author: xeioex

.github/workflows/check-pr.yml

@@ -3,6 +3,10 @@ name: check-pr
 on:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/master' }}
+
 jobs:
   build:
     runs-on: [ ubuntu-24.04 ]