CI: Cancel any in-progress job or run (#2930)

See https://docs.github.com/en/actions/using-jobs/using-concurrency

.github/workflows/main.yml

@@ -1,5 +1,9 @@
 name: CI
 
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
 on:
   push:
   pull_request: