snapshot: Does not mount /.snapshots correctly with Debian installer created btrfs

https://discuss.freedombox.org/t/failed-failed-to-mount-snapshots-bullseye/1449/7

When btrfs filesystem is created by the Debian installer and freedombox is installed on that machine, the machine fails to boot.

This is because in a normal FreedomBox image, the snapshot is at the path .snapshots but in Debian installer step the snapshot is at @rootfs/.snapshot