Reduce automatic CI build frequency to once every 9 days

Building every 7 days causes the cached data to be retained:
<https://github.com/actions/cache/tree/v2.1.6#cache-limits>.

.github/workflows/ci.yaml

@@ -5,7 +5,7 @@ on:
   pull_request:
   push:
   schedule:
-    - cron:  '45 15 * * 1'
+    - cron:  '45 15 */9 * *'
 
 permissions:
   contents: read