Disable controller mode for .NET Core.

This is required until the required APIs are present in .NET Core (NamedPipes security).

src/Uno.SampleCoreApp/Uno.SampleCoreApp.csproj

@@ -3,7 +3,6 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFramework>netcoreapp2.1</TargetFramework>
-	<UnoSourceGeneratorUseGenerationHost>true</UnoSourceGeneratorUseGenerationHost>
   </PropertyGroup>
 
   <PropertyGroup>

src/Uno.SourceGeneratorTasks.Dev15.0/Tasks/SourceGenerationTask.cs

@@ -148,10 +148,10 @@ namespace Uno.SourceGeneratorTasks
 
 		public bool SupportsGenerationController
 			=> (bool.TryParse(UseGenerationController, out var result) && result)
+			&& !RuntimeHelpers.IsNetCore
 			&& (
 				!RuntimeHelpers.IsMono
 				|| RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
-				|| RuntimeHelpers.IsNetCore
 			);