Only use the targeting pack nuget packages on non-windows

Ideally we would use the .NET framework targeting pack references
everywhere but right now we are hitting error:

: error MSB3245: Could not resolve this reference. Could not locate the assembly "System.Runtime". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors.

github issue dotnet/installer#2455

eng/Directory.Build.Data.targets

@@ -11,7 +11,7 @@
     <Error Condition="'@(RequiredTargetFrameworks)' == ''" Text="List of required target frameworks is empty something must have messed up property RequiredTargetFrameworks[$(RequiredTargetFrameworks)]." />
     <Error Condition="'@(MissingTargetFrameworks)' != ''" Text="Missing required target frameworks '@(MissingTargetFrameworks)'. Please ensure you add those frameworks." />
   </Target>
-  
+
   <!-- Set PackageProjectUrl to the package README for Client Libraries -->
   <Target Name="SetPackageProjectUrl" BeforeTargets="ValidateTargetFrameworks" Condition="'$(IsClientLibrary)' == 'true' and '$(IsTestProject)' != 'true' and
    '$(IsSamplesProject)' != 'true' and '$(IsTestSupportProject)' != 'true' and '$(CommitSHA)' != ''">
@@ -26,7 +26,7 @@
   <!-- This allows us to build .NET Framework targets on non-windows
     TODO: Move the NETFramework reference assemblies to a feed other then the roslyn feed.
   -->
-  <ItemGroup Condition="'$(IsTargetingNetFx)' == 'true'">
+  <ItemGroup Condition="'$(IsTargetingNetFx)' == 'true' and '$(OS)' != 'Windows_NT'">
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="all" IsImplicitlyDefined="true" />
   </ItemGroup>