Add CI schedule

Author: thinknathan

.github/workflows/bob.yml

@@ -5,6 +5,8 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  schedule:
+    - cron: '0 2 2 */4 *'
   workflow_dispatch:
 env:
   BUILD_SERVER: 'https://build.defold.com'