Update build workflow schedule and triggers.

PiperOrigin-RevId: 874177472

Author: Orbax Authors

.github/workflows/build.yml

@@ -3,15 +3,12 @@ name: build
 on:
   # continuous
   schedule:
-    # Run every hour
-    - cron: "0 * * * *"
+    # Run every 4 hour
+    - cron: "0 */4 * * *"
   push:
     branches:
       - main
       - 'test_*'
-  pull_request:
-    branches:
-      - main
 
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   contents: read