Revert. Don't always build wheels in PRs

Author: eriknw

.github/workflows/wheels.yml

@@ -1,7 +1,7 @@
 name: Build wheels
 
 on:
-  pull_request:  # Uncomment to debug wheel building in a PR
+  # pull_request:  # Uncomment to debug wheel building in a PR
   push:
     branches:
       - master