bug#73903: [shepherd]: Guix Home's shepherd masks shutdown/reboot on foreign distribution

[Dariqq]

When using guix home to manage shepherd services on a foreign 
distribution the shepherd package is also added to the home-profile.

This then makes the home profile provide 'halt', 'shutdown' and 'reboot' 
(and their manpages) masking the system ones.

Trying to run one of these then results in an error:
reboot: error: /var/run/shepherd/socket: No such file or directory

instead of e.g. rebooting the system.