bug#44956: Rebooting or powering down UEFI machine locks it up

[guixuser6392 via Bug reports for GNU Guix]

Further investigation shows that this regression does not affect either the 
shutdown process or the reboot one on GNU/Linux distributions that adopted the 
systemd init system. This may indicate that no attempt to automatically unmount 
the efivars file system is ever made on those, which raises the question: why 
do we explicitly unmount this file system in the first place?