Change build scripts from Nuget.exe to DotNet.exe

[AdamBraden]

nuget.exe is a separate download. Since we are building cswinrt projections, most folks will have the dotnet sdk installed. Switching over to dotnet.exe will avoid having an extra dependency that you need to download and place somewhere in the path

[Scottj1s]

dotnet restore doesn't appear to support packages.config projects (C++), so I think nuget is still required.

dotnet/sdk#7922

[jkoritzinsky]

Additionally, the dotnet CLI doesn't support C++ projects, so it wouldn't be able to restore the C++/WinRT projects' packages anyway.

[Scottj1s]

Otoh, we can certainly download nuget from the script to address that as a separate step