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

[corngood]

The use of $(NuGetPackageRoot) here:

https://github.com/tunnelvisionlabs/ReferenceAssemblyAnnotator/blob/2edad1afcd996e2cd54fb18279326e935818f0f6/TunnelVisionLabs.ReferenceAssemblyAnnotator/TunnelVisionLabs.ReferenceAssemblyAnnotator.targets#L80

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.