Beta Releases Point to the Wrong Location

generate_appcast will default to setting the release notes and download link relative to the feedURL from the bundle. However, if that URL is set the releases, betas won't use the correct URL. The AppCast needs to be updated manually (which isn't a great solution, or applications need to know they are betas at build time.

If that's the case UpdateServer needs to know where a build can be copied, which means it needs to know what URL it's serving.