Use library-packs also when RestoreAdditionalProjectSources is explicitly set. (#44863)

Author: dsplaisted

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.NuGetOfflineCache.targets

@@ -9,7 +9,7 @@ WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and
 Copyright (c) .NET Foundation. All rights reserved.
 ***********************************************************************************************
 -->
-<Project ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project TreatAsLocalProperty="RestoreAdditionalProjectSources" ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
   <PropertyGroup>
     <_NugetFallbackFolder>$(MSBuildThisFileDirectory)..\..\..\..\NuGetFallbackFolder</_NugetFallbackFolder>