Automatically cancel outdated CI runs on PR

Author: taiki-e

.github/workflows/ci.yml

@@ -29,6 +29,10 @@ defaults:
   run:
     shell: bash
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   tidy:
     uses: taiki-e/workflows/.github/workflows/tidy.yml@main