Revert "run no-xamarin build/test on linux as well"

This reverts commit ffaa8df.

Author: ericsink

.github/workflows/dotnet.yml

@@ -11,7 +11,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-24.04, windows-2025, macos-15]
+        os: [windows-2025, macos-15]
 
     runs-on: ${{ matrix.os }}