Make impermanence reset work on unencrypted hosts.

common/impermanence.nix

@@ -64,7 +64,7 @@ in
     # Note `lib.mkBefore` is used instead of `lib.mkAfter` here.
     boot.initrd.postDeviceCommands = pkgs.lib.mkBefore ''
       mkdir /mnt
-      mount /dev/mapper/luksroot /mnt
+      mount ${config.fileSystems."/".device} /mnt
       if [[ -e /mnt/root ]]; then
           mkdir -p /mnt/old_roots
           timestamp=$(date --date="@$(stat -c %Y /mnt/root)" "+%Y-%m-%-d_%H:%M:%S")