feat: only one CI job at a time per PR

This prevents the Gitea runner from getting clogged up with stale CI checks.

.gitea/workflows/ci.yml

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