Run GitHub Actions continuously

Summary:
Schedule test runs every 2 hours, starting at 30 minutes past the hour.
Under heavy load, builds may be gracefully delayed or skipped, so
having too short of an interval shouldn't cause any disruptions.

Differential Revision: D61042384

.github/workflows/build.yml

@@ -3,6 +3,9 @@ on:
   pull_request:
     branches:
       - main
+  schedule:
+    - cron: '30 */2 * * *'
+
 jobs:
   android:
     runs-on: ubuntu-latest