Only check for broken Ubuntu .NET SDK install on Linux systems

fixes 697cf25

Author: YoshiRulz

src/MainSlnCommon.props

@@ -51,7 +51,7 @@
 		<Compile Remove="$(MSBuildProjectDirectory)/debug/**/*" />
 	</ItemGroup>
 	<Target Name="UbuntuMispackagedSDKCheck" BeforeTargets="Build">
-		<Error Condition=" !Exists('$(MSBuildSDKsPath)/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.targets') "
+		<Error Condition=" '$(OS)' != 'Windows_NT' && !Exists('$(MSBuildSDKsPath)/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.targets') "
 			Text="Microsoft.NET.Sdk.WindowsDesktop is missing—if you're on Ubuntu, DON'T install the .NET SDK from the official repos" />
 	</Target>
 </Project>