Use NuGet.config at root so VS can see it

See also https://github.com/dotnet/corefx/pull/233

Once that is in and this is adopted in a new buildtools version, we can delete the copy in the nuget/ sub-folder.
This is now set in the dir.props file so we can actually just delete this property from this targets file along with the command line property. I will do that as part of my clean-up in the build tools repo. Feel free to go ahead and close this PR.