ci: prevent concurrent runs with concurrency group

Author: shimomo

.github/workflows/cron.yml

@@ -5,6 +5,10 @@ on:
     - cron: '0,30 * * * *'
   workflow_dispatch:
 
+concurrency:
+  group: cron-${{ github.ref }}
+  cancel-in-progress: false
+
 permissions:
   contents: write