NuGetUpdateSource is missing in NuGetSources

[ronaldvanmanen]

Hi,

I searched the sources, issues, and discussions for NuGetUpdateSource but could not find anything. Anyway, it seems that the class Cake.Common.Tools.NuGet.Sources.NuGetSources is missing method NuGetUpdateSource(string name, string source, NuGetSourcesSettings settings). The implementation of this method should be pretty straight forward. I'd be happy to implement it and send in a pull request.

I've got a branch ready for PR @ https://github.com/ronaldvanmanen/cake/tree/feature/4230.