bug#39366: guix offload sometimes can use ~/.ssh/config, sometimes not

[Maxim Cournoyer]

"hurd" is a host name defined in my ~/.ssh/config file.

$ guix offload status
guix offload: getting status of 1 build machines defined in 
'/etc/guix/machines.scm'...
guix offload: warning: machine 'hurd' is 31553985 seconds behind
load on machine 'hurd' is 0.02 (normalized: 0.02)
hurd
  kernel: Linux 5.1.2-gnu
  architecture: x86_64
  host name: hurd
  normalized load: 0.02
  free disk space: 950819.34 MiB
  time difference: -31553985 s

$ guix offload test
guix offload: testing 1 build machines defined in '/etc/guix/machines.scm'...
guix offload: Guix is usable on 'hurd' (test returned 
"/gnu/store/883yjkl46dxw9mzykykmbs0yzwyxm17z-test")
guix offload: 'hurd' is running GNU Guile 2.2.4
sending 1 store item (0 MiB) to '10.42.0.199'...
exporting path `/gnu/store/s9f3igal64f8c4k23jda5z24pfpzqxv6-export-test'
guix offload: 'hurd' successfully imported 
'/gnu/store/s9f3igal64f8c4k23jda5z24pfpzqxv6-export-test'
retrieving 1 store item from '10.42.0.199'...
guix offload: successfully imported
'/gnu/store/y7l8zymvqcnja6ybk8lzxncf9v0r5g79-import-test' from 'hurd'

$ guix build git --no-substitutes --check
process 31348 acquired build slot '/var/guix/offload/hurd:22/0'
guix offload: error: failed to connect to 'hurd': Failed to resolve hostname 
hurd (Name or service not known)
waiting for locks or build slots...

So, in many cases it seems able to use the information in ~/.ssh/config
to connect with the machine, but not when actually trying to offload
builds.

Maxim