bug#27388: offloading: Add support for keyfile-less keys as used by GnuPG

[ng0]

At the moment the field (private-key) in /etc/guix/machines.scm expects
to be a file.
When you use GnuPG authentication keys for ssh logins, you have no
pubkey file, but you have a very long pubkey which can be used with
~/.ssh/authorized_keys and similar mechanisms.

Example:

address@hidden ~/src/guix/guix$ cat /etc/guix/machines.scm
(list (build-machine
      …
       (privat-key "ssh-rsa 
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
 (none)")
      …
-- 
ng0
OpenPG: A88C8ADD129828D7EAC02E52E22F9BBFEE348588
https://krosos.org/~/ng0/ https://www.infotropique.org

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