bug#31825: guix offload fails with guix-authenticate error

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Just as a follow-up; I've managed to fall into this trap again,
attempting to authorize the keys by adding them to the 'authorize-keys'
field of guix-configuration record.

On the local machine:

--8<---------------cut here---------------start------------->8---
guix offload test /etc/guix/machines.scm 127.0.0.1
guix offload: testing 1 build machines defined in '/etc/guix/machines.scm'...
guix offload: Guix is usable on '127.0.0.1' (test returned "/gnu/store/883yjkl46dxw9mzykykmbs0yzwyxm17z-test")
guix offload: '127.0.0.1' is running GNU Guile 3.0.0
sending 1 store item (0 MiB) to '127.0.0.1'...
exporting path `/gnu/store/l9mph3k5l26nm8mb50imsklbsz0bji0b-export-test'
guix offload: error: program `/gnu/store/amjsgks2n05k9lkck78z64nphad1dkqr-guix-1.0.1-13.50299ad/bin/guix' failed with exit code 1
--8<---------------cut here---------------end--------------->8---

On the remote machine:

--8<---------------cut here---------------start------------->8---
sudo strace -p 15683 -p 15716 -f -s345 -o /tmp/log
--8<---------------cut here---------------end--------------->8---

And found within /tmp/log:

16120 write(2, "guix authenticate: error: error: unauthorized public key: (public-key \n (ecc \n  (curve Ed25519)\n  (q #MY-PUBLIC-KEY#)\n  )\n )\n", 176) = 176

So, still actual :-)

Maxim