fix: Force use Uno's Roslyn hosted generators instead of Uno.SourceGeneration tasks (#2199)

samples/Directory.Build.props

@@ -5,4 +5,12 @@
     <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="3.9.0" PrivateAssets="all" />
   </ItemGroup>
 
+  <PropertyGroup>
+    <!-- 
+    Force use Uno's Roslyn hosted generators instead of Uno.SourceGeneration tasks, even when LangVersion below 9.0.
+    The original issue is caused by https://github.com/unoplatform/uno/issues/9297
+    -->
+    <UnoUIUseRoslynSourceGenerators>true</UnoUIUseRoslynSourceGenerators>
+  </PropertyGroup>
+
 </Project>