Enable x-plat net46 build using framework reference assemblies from NuGet

Author: nguerrera

build/Versions.props

@@ -27,6 +27,11 @@
     <SystemReflectionMetadataVersion>1.5.0</SystemReflectionMetadataVersion>
   </PropertyGroup>
 
+  <!-- Get .NET Framework reference assemblies from NuGet packages -->
+  <PropertyGroup>
+    <UsingToolNetFrameworkReferenceAssemblies>true</UsingToolNetFrameworkReferenceAssemblies>
+  </PropertyGroup>
+
   <!-- Test Dependencies -->
   <PropertyGroup>
     <FluentAssertionsVersion>4.19.2</FluentAssertionsVersion>

src/Tasks/Microsoft.NET.Build.Extensions.Tasks/Microsoft.NET.Build.Extensions.Tasks.csproj

@@ -10,7 +10,6 @@
     <PackageId>Microsoft.NET.Build.Extensions</PackageId>
     <RootNamespace>Microsoft.NET.Build.Tasks</RootNamespace>
     <TargetFrameworks>netcoreapp2.0;net46</TargetFrameworks>
-    <TargetFrameworks Condition="'$(OS)' != 'Windows_NT'">netcoreapp2.0</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup>

src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj

@@ -7,7 +7,6 @@
     <OutputType>Library</OutputType>
     <PackageId>Microsoft.NET.Sdk</PackageId>
     <TargetFrameworks>netcoreapp2.0;net46</TargetFrameworks>
-    <TargetFrameworks Condition="'$(OS)' != 'Windows_NT'">netcoreapp2.0</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup>