This patch mirrors a similar change in nixpkgs [1][2].

The official method for downloading from SourceForge is to use
downloads.sourceforge.net, which redirects to various other mirrors. It
can be unreliable, but also faster because it can find a nearby mirror
and load-balance. If it fails Guix will still fall back to known good
mirrors.

[1]: https://github.com/NixOS/nixpkgs/issues/16900
[2]: https://github.com/NixOS/nixpkgs/pull/16976
-- 
Alex Griffin