Add LabsFeed source inline instead of using nuget.config

[Arlodotexe]

This PR fixes errors we're seeing in CI here in the "Push packages (main)" step:

Run dotnet nuget update source LabsFeed `
error: Object reference not set to an instance of an object.
error: The specified source 'LabsFeed' is invalid. Provide a valid source.
Error: Process completed with exit code 1.

This regression was introduced in #663