GA: schedule the build action to run every month

it has the issue that the action will get disabled after 60d inactivity,
but public comments probably count towards the activity, so it might still help with artifact retention

.github/workflows/build.yaml

@@ -5,6 +5,8 @@ on:
   pull_request:
     branches: [ "main" ]
   workflow_dispatch:
+  schedule:
+    - cron: '46 5 7 * *' # At 05:46 on day-of-month 1 in every 3rd month from January through December
 
 jobs:
   build-rust: