Run "rebuild libs cache" every day 3 hours before nightlies run (#4386)

Usually this will quickly exit, however, if anything is out of date
3 hours before nightlies, it will probably be out of date at the time
that nightlies happen. Running ahead of time will make the release
more efficient. That efficiency is nice because if we hit a problem
elsewhere with the release, it will happen "sooner in the evening" with
more time for someone to look into it.

.github/workflows/update-lib-cache.yml

@@ -1,7 +1,9 @@
 name: Manually update lib cache
 
 on:
-  workflow_dispatch
+  workflow_dispatch:
+  schedule:
+    - cron: "0 21 * * *"
 
 concurrency:
   group: "update-lib-cache"