ci: Automatically cancel CI runs if we push a new version to the PR

We often have a number of changes in flight, and we amend PRs before the previous run completes. This allows us to cancel that prior run, preserving our precious runners.

.github/workflows/ci.yaml

@@ -6,6 +6,10 @@ on:
       - "master"
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+  
 jobs:
   prebuild:
     name: Pre-build checks