Specifies the server URL. Supported URLs for nuget.org include https://www.nuget.org,
https://www.nuget.org/api/v3, and https://www.nuget.org/api/v2/package. For private feeds, replace the
host name (for example, %hostname%/api/v3).
This option is required unless DefaultPushSource config value is set in the NuGet config file.