ci: cancel superseded PR builds (#34842)

### Reason for this change

Pushing a new commit should cancel any in progress PR builds.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: mrgrain

.github/workflows/pr-build.yml

@@ -12,6 +12,12 @@ on:
       - main
       - v2-release
 
+# For every PR, cancel any previous builds in progress
+# ... but for all other builds we keep them running
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event.pull_request.number || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: aws-cdk_ubuntu-latest_32-core