When user uses --packages $path, the $path should be sent to dotnet build command as well

[adamsitnik]

In dotnet/performance#594 I have hit an issue in a CI script that was running following command:

dotnet restore --packages $path
dotnet build --no-restore

It turns out, that if we pass the packages restore folder to dotnet restore, we should also pass it to dotnet build via NuGetPackageRoot magic MSBuild property ;)