isolate pushes to only main for builds

.github/workflows/build.yml

@@ -5,6 +5,8 @@ on:
         "00 06 * * *" # build at 06:00 UTC every day
         # (20 minutes after last ublue images start building)
   push:
+    branches:
+      - main
     paths-ignore: # don't rebuild if only documentation has changed
       - "**.md"