Updating Pull Requests should cancel previous runs (#432)

When iterating on PRs sometimes we queue up a large number of test runs.
We only really care about the last of these, so arrange for later runs
to cancel any run in progress using the technique outlined in
https://stackoverflow.com/a/72408109

.github/workflows/fastlane-tests.yml

@@ -3,6 +3,10 @@ name: Fastlane Tests
 on:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: macos-13