Add push: { branches: [main] } so it gets executed once

There seem to be a limitation with workflow_dispatch where job doesn't appear if not triggered at least once. I wonder how this can be solved. 🤔

.github/workflows/manual_activation.yml

@@ -1,5 +1,6 @@
 name: Acquire activation file
 on:
+  push: { branches: [main] }
   workflow_dispatch:
     inputs:
       unityVersion: