chore(ci): cancel superseded PR workflow runs

Add a concurrency group keyed by PR number with cancel-in-progress
enabled, so pushing new commits to a PR cancels any still-running
checks from earlier commits. Saves runner minutes and gives quicker
feedback on the latest commit.

Author: einarwar

.github/workflows/on-pull-request.yaml

@@ -3,6 +3,10 @@ on:
   pull_request:
     types: [opened, reopened, synchronize]
 
+concurrency:
+  group: pr-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   linting-and-checks:
     name: "Linting and checks"