bug#34041: guix substitute 504 gateway error retrieving python-numpy

[maxim . cournoyer]

The full error reads as:

--8<---------------cut here---------------start------------->8---
guix substitute: error: download from 
'https://mirror.hydra.gnu.org/guix/nar/gzip/vki33vwz46zpm13bm97gldr91hywpjkw-python-numpy-1.15.4'
 failed: 504, "Gateway Time-out"
substitution of
/gnu/store/vki33vwz46zpm13bm97gldr91hywpjkw-python-numpy-1.15.4 failed
--8<---------------cut here---------------end--------------->8---