backup: fix bug in adding backup location
This commit provides a fix for the issue described here:
https://discuss.freedombox.org/t/how-do-i-restore-freedombox/2022/10
Test:
- Vagrant up, create a backup on a usb stick
- Rebuild the VM, then try to add the backup location; confirm the issue still exists.
- Apply the code and confirm the previously unknown location can be added
Signed-off-by: Benedek Nagy contact@nbenedek.me
Edited by Benedek Nagy