guix: add distro

Hi,

I'm a contributor to GNU Guix, a distribution that is not yet in the osinfo database, and this commit adds it. I have to say, I mostly copied and tried to follow what other files were doing, but I couldn't find the documentation for the format, so it might be missing something. One point on which I'm less than confident is that Guix provides its ISO in a compressed format. I've put the .iso.xz link in the xml file for now. Please let me know if this could cause issues, we could discuss it and provide a bare .iso download link.