Thought $default-branch is a real constant in Github.

.github/workflows/main.yml

@@ -5,9 +5,9 @@ name: Python application
 
 on:
   push:
-    branches: [ $default-branch ]
+    branches: [ master ]
   pull_request:
-    branches: [ $default-branch ]
+    branches: [ master ]
 
 permissions:
   contents: read