Run test CI only on push to master or PR to master

Author: finsberg

.github/workflows/test_package.yml

@@ -4,7 +4,7 @@ on:
   push:
     # The CI is executed on every push on every branch
     branches:
-      - "**"
+      - main
   pull_request:
     # The CI is executed on every pull request to the main branch
     branches: