github: cancel concurrent workflows for the same PR/push

.github/workflows/main.yml

@@ -8,6 +8,11 @@ on:
     branches:
       - "*"
 
+concurrency:
+  # Cancel any previous workflows if they are from a PR or push.
+  group: ${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 defaults:
   run:
     shell: bash