Allow nuget.config probing start path to be able to set in project file

[wli3]

Tools CLI ask

[emgarten]

@wli3 as a workaround until this is added you might be able to use RestoreSolutionDirectory

Relevant code is here:

• https://github.com/NuGet/NuGet.Client/blob/572962f1b1ee890533e5a75163264bd021426dc7/src/NuGet.Core/NuGet.Build.Tasks/GetRestoreSettingsTask.cs#L115
• https://github.com/NuGet/NuGet.Client/blob/572962f1b1ee890533e5a75163264bd021426dc7/src/NuGet.Core/NuGet.Build.Tasks/RestoreSettingsUtils.cs#L19

[nkolev92]

Added RestoreRootConfigDirectory