ensure superceded PR workflows are cancelled

Add a concurrency specifier so that github actions cancels matching PR
pushes and similar, saves a lot of compute when iterating on commit
style and similar with force pushes.

.github/workflows/main.yml

@@ -1,5 +1,8 @@
 on: [ pull_request, push, workflow_dispatch ]
 name: ci
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 jobs:
   check-pr:
     name: validate commits