annotator is broken when microsoft.netcore.app.ref isn't in NuGetPackageRoot

[corngood]

The use of $(NuGetPackageRoot) here:

ReferenceAssemblyAnnotator/TunnelVisionLabs.ReferenceAssemblyAnnotator/TunnelVisionLabs.ReferenceAssemblyAnnotator.targets

Line 80 in 2edad1a

<_AnnotatedReferenceAssemblyDirectoryContents Include="$(NuGetPackageRoot)\microsoft.netcore.app.ref\$(AnnotatedReferenceAssemblyVersion)\ref\*\*.dll" />

obviously doesn't work if microsoft.netcore.app.ref isn't in the package root, but this is possible when using fallback package paths.

I think ideally this should be getting the path as resolved by nuget, but I'm not quite sure how to go about doing that.