Allow cancellation of PR jobs

.github/workflows/ci.yml

@@ -1,10 +1,17 @@
 name: CI
+
 on:
   push:
     branches:
       - main
   pull_request:
 
+# Cancel in progress workflows on pull_requests.
+# https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 # Needed for nx-set-shas when run on the main branch
 permissions:
   actions: read