Unescape PackageConflictPreferredPackages

The MSBuild engine automatically escapes strings returned from tasks,
which was causing problems because the later task
`ResolvePackageFileConflicts` was expecting to be passed an array of
strings, rather than an array consisting of a single string with a bunch
of semicolons in it.

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.FrameworkReferenceResolution.targets

@@ -369,6 +369,12 @@ Copyright (c) .NET Foundation. All rights reserved.
 
     </ResolveTargetingPackAssets>
 
+    <PropertyGroup>
+      <!-- Unescape this semicolon-delimited ordered list so that it's passed as a list to targets
+           that consume it. -->
+      <PackageConflictPreferredPackages>$([MSBuild]::Unescape($(PackageConflictPreferredPackages)))</PackageConflictPreferredPackages>
+    </PropertyGroup>
+
     <ItemGroup Condition="'$(RuntimeIdentifier)' == '' or '$(SelfContained)' != 'true'">
       <PackageConflictPlatformManifests Include="@(PlatformManifestsFromTargetingPacks)" />
     </ItemGroup>