re-enable ci workflow on main (#1141)

It is used to update the cache with a fresh copy, and prevent it from
getting bigger over time.

.github/workflows/ci.yml

@@ -8,10 +8,8 @@ on:
   # Run on every pull request:
   pull_request: {}
 
-  # Run on pushes to any branch except 'main', as we use a merge queue, and test PRs individually before merging them:
-  push:
-    branches-ignore:
-      - "main"
+  # Run on pushes to any branch even 'main', as it is used to refresh the CI cache from scratch, so that it doesn't grow indefinitely:
+  push: {}
 
 # In the event that there is a new push to the ref, cancel any running jobs because they are now obsolete, wasting resources.
 concurrency: