Revert "Run on all pushes"

There are too many pushes for this to be a good idea.  We now have
workflow_dispatch so we can manually launch jobs instead.

This reverts commit e765670.

.github/workflows/ci.yml

@@ -4,6 +4,7 @@ on:
   pull_request:
     types: [synchronize, opened, reopened]
   push:
+    branches: [main]
   schedule:
     # additionally run once per week (At 00:00 on Sunday) to maintain cache
     - cron: '0 0 * * 0'