ci: cancel in-progress GH runs for same PR || ref

If you're pushing new work it's generally not useful to have CI churning
on the old work. Cancel it and get cooking on the new stuff ASAP.

See github docs for more info:

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#example-concurrency-groups

.github/workflows/ci.yml

@@ -10,6 +10,10 @@ on:
   schedule:
     - cron: '0 21 * * *'
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Build + test