Explicitly run tests on main branch. (#4)

Having a few issues running the tests when pushing directly to `main`, so maybe this will help.

.github/workflows/test.yml

@@ -2,9 +2,9 @@ name: Run tests
 
 on:
   push:
-    branches: [ $default-branch ]
+    branches: [ main ]
   pull_request:
-    branches: [ $default-branch ]
+    branches: [ main ]
 
 jobs:
   build: