Only run windows installer test on main (condition was removed in #241)

Author: RalfG

.github/workflows/test.yml

@@ -45,6 +45,8 @@ jobs:
         run: uv run ms2rescore --help
 
   test-windows-installer:
+    # Only run on push to main (e.g., after PR merge)
+    if: ${{ github.ref == 'refs/heads/main' }}
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v4