nixos/borgbackup: add option to allow removable devices

When having backup jobs that persist to a removable device like an
external HDD, the directory shouldn't be created by an activation script
as this might confuse auto-mounting tools such as udiskie(8).

In this case the job will simply fail, with the former approach
udiskie ran into some issues as the path `/run/media/ma27/backup` was
already there and owned by root.