chore: cancel previous GHA from the same PR (#745)

.github/workflows/ci.yaml

@@ -11,6 +11,11 @@ on:
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch:
 
+concurrency:
+  # Cancel previous actions from the same PR: https://stackoverflow.com/a/72408109
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   # matrix-prep-* steps dynamically generate a bit of JSON depending on whether our action has
   # access to repository secrets. When running on a pull_request from a fork, the author is