bug#36659: There should be an unattended upgrades service

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

Some users (want to) forget about regularly upgrading Guix System.
There should be an unattended upgrades service.  Some requirements
come to mind for its configuration:

1) Some users may want their unattended upgrades service to take care
just of reconfiguring from a recent checkout and some may want it to
take care of updating users’ ~/.config/guix/current and ~/guix-profile
profiles.

2) Maybe there should be libnotify integration for unattended
upgrades if the user uses a desktop environment.

3) Updates may fail if there is no internet connection.  Some users
may *not* want upgrades on metered internet connections.  Some users
may *not* want upgrades over untrusted connections.

This report is a followup to Ludo’s proposal at
<https://issues.guix.gnu.org/issue/36636> to add such a service and
add it to %desktop-services, making it the default setting.  Such a
change in defaults could be a bad surprise for some users and should
not go unnoticed, I think.

Regards,
Florian