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

[maxim . cournoyer]

Another one: 
https://mirror.hydra.gnu.org/guix/nar/gzip/7rm5mk8v91dkg40gmz5lsvq7kqfpf8l3-mercurial-4.7.2'
 failed: 504, "Gateway Time-out"