made image rebuild on a schedule instead of squashing

Author: netbrain

.github/workflows/zwift_build_from_scratch.yaml

@@ -1,6 +1,8 @@
 name: Zwift build from scratch
 on:
   workflow_dispatch:
+  schedule:
+    - cron: "0 0 1 */3 *"    
 jobs:
   zwift_build:
     runs-on: ubuntu-22.04