Fix LT-22220: Copy Nuget System.Buffers post build

* The GAC version of System.Buffers was being copied during
  project builds making the version of Visual Studio on the
  build system determine what version of Buffers ended up
  in the Build Output. Rather than add a direct reference
  to every project that might pull it in as a tertiary
  dependency add a post build copy

Author: jasonleenaylor

Src/Directory.Build.targets

@@ -1,14 +1,18 @@
 <Project>
-<!-- .Net Framework projects which reference netstandard2.0 end up copying a bad version of System.ValueTuple -->
+<!-- .Net Framework projects which reference netstandard2.0 end up copying a bad version of some System libraries -->
 <!-- Rather than try to fix it here and there we'll just go nuclear and copy the right one in after every project -->
-	<Target Name="CopyCorrectSystemValueTuple" AfterTargets="Build">
+	<Target Name="CopyCorrectSystemAssemblies" AfterTargets="Build">
 		<ItemGroup>
-			<TupleAssembly Include="$(MSBuildThisFileDirectory)/../packages/System.ValueTuple.4.5.0/lib/net461/System.ValueTuple.dll" />
+			<SystemAssembly Include="$(MSBuildThisFileDirectory)/../packages/System.ValueTuple.4.5.0/lib/net461/System.ValueTuple.dll" />
+			<SystemAssembly Include="$(MSBuildThisFileDirectory)/../Downloads/System.Buffers.dll" />
 		</ItemGroup>
-		<Message Text="Copying @(TupleAssembly) to $(OutDir)" />
+		<Message Text="Copying @(SystemAssembly) to $(OutDir)" />
 		<Error
 			Text="System.ValueTuple not found in the packages folder."
 			Condition="!Exists('$(MSBuildThisFileDirectory)/../packages/System.ValueTuple.4.5.0/lib/net461/System.ValueTuple.dll')" />
-		<Copy SourceFiles="@(TupleAssembly)" DestinationFolder="$(OutDir)" />
+		<Error
+			Text="System.Buffers not found in the downloads folder."
+			Condition="!Exists('$(MSBuildThisFileDirectory)/../Downloads/System.Buffers.dll')" />
+		<Copy SourceFiles="@(SystemAssembly)" DestinationFolder="$(OutDir)" SkipUnchangedFiles="true" />
 	</Target>
 </Project>