Cancel workflows when they become outdated

This change ensures that workflows are canceled when new changes come in which means resources aren't wasted on outdated workflows.

More information: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency

.github/workflows/ci.yml

@@ -13,6 +13,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ci-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   preflight:
     runs-on: ubuntu-latest