Restore the NuGet tool when no existing executable can be found

[AdmiringWorm]

When trying to call any of the NuGet aliases when running under .NET Core (or if Cake haven't been installed through nuget, or no nuget.exe exist on the system), an exception happens when trying to run any of the NuGet aliases (NuGetPush, NuGetPack, and so on).

I would suggest to implement the following:

Before trying to run any of these aliases, make a check using Context.Tool.Resolve and if it is null, try restoring the NuGet.CommandLine nuget package.