Only run one review-apps workflow at a time

This prevents us attempting to run multiple instance of terraform at
the same time. Instead, the current running workflow will complete
before the next one starts.

Only 1 running and 1 pending workflow is allowed - any further
workflows will supercede the pending one.

Author: whi-tw

.github/workflows/review_apps_on_pr_change.yml

@@ -5,6 +5,10 @@ on:
     # matches the docs for the default types
     # https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#pull_request
     types: [opened, reopened, synchronize]
+
+concurrency:
+  group: "review-apps-forms-runner-pr-${{ github.event.pull_request.number }}"
+  cancel-in-progress: false
 jobs:
   update-review-app:
     runs-on: ubuntu-24.04-arm

.github/workflows/review_apps_on_pr_close.yml

@@ -3,6 +3,10 @@ on:
   pull_request:
     # only run when a PR is closed or merged
     types: [closed]
+
+concurrency:
+  group: "review-apps-forms-runner-pr-${{ github.event.pull_request.number }}"
+  cancel-in-progress: false
 env:
   IMAGE_TAG: "842676007477.dkr.ecr.eu-west-2.amazonaws.com/forms-runner:pr-${{github.event.pull_request.number}}-${{github.event.pull_request.head.ref}}"
 jobs: