Set CI source URLs for prerelease packages

[ekohl]

This allows regular tools, like spectool, to retrieve the sources which makes development easier. Once the build changes to rc1 or actual releases the old URL is used automatically.

Right now it still does fetching in the custom releaser, but this is aimed at starting a discussion on how to simplify that.

The biggest thing that's still desired is to set a proper version, including which git commit was used. That's useful for tracking and makes builds unique.