I cannot build xz with guix 0.8.3:

$ guix build xz
...
ERROR: download failed "http://tukaani.org/xz/xz-5.0.4.tar.gz" 403
"Forbidden"
...

I'm not sure if this can be solved by contacting the admin of that web, but
I guess that bumping xz version to at least 5.0.8 (I checked it was
possible to download this one) could fix this problem.

-- 
Jan Synáček