Fix package source name

[werwolfby]

Default global nuget.config uses nuget.org as key.

In my case this global nuget.config has private source, and special packageSourceMapping to use my private source for some packages. And use nuget.org as default one for everything else:

  <packageSourceMapping>
    <packageSource key="nuget.org">
      <package pattern="*" />
    </packageSource>
    <packageSource key="werwolf.by">
      <package pattern="werwolfby.*" />
    </packageSource>
  </packageSourceMapping>

Because nuget.config specified api.nuget.org as default, my global one doesn't allow me to restore packages in nuke project. This fix returns everything how it should be.

There is the question if nuke requires nuget.config at all?

I confirm that the pull-request:

• Follows the contribution guidelines
• Is based on my own work
• Is in compliance with my employer

[gtbuchanan]

There is the question if nuke requires nuget.config at all?

I'll second this question. I also ran into this issue since I have package source mapping configured in my user-level NuGet.Config. I deleted NuGet.Config locally from the Nuke repository to move forward with my PR, but I suppose this rename would have worked as well.