bug#21010: eject fails because of hardcoded path to umount

[Alex Sassmannshausen]

Hello,

I'm running GuixSD.

I tried to 'eject' a pen drive today using
`sudo eject /mnt/usb`

The command failed with 
-------8<---------------->8-----------
eject: unable to exec /bin/umount of `/mnt/usb'
eject: unmount of `/mnt/usb' failed
-------8<---------------->8-----------

Some more information:
-------8<---------------->8-----------
# which eject
/run/current-system/profile/bin/eject
# mount /dev/sdb /mnt/usb
# eject /mnt/usb
eject: unable to exec /bin/umount of `/mnt/usb'
eject: unmount of `/mnt/usb' failed
-------8<---------------->8-----------

HTH, let me know if you want me to test or provide anything else.

Cheers,
Alex