nixos/lxd: add missing util-linux to $PATH

This fixes `lxd init`, which previously failed like this:

  $ yes "" | lxd init
  [...]
  Error: Failed to create storage pool "default": Failed to run: losetup --find --nooverlap --direct-io=on --show /var/lib/lxd/disks/default.img: exec: "losetup": executable file not found in $PATH

nixos/modules/virtualisation/lxd.nix

@@ -140,7 +140,8 @@ in {
       ];
       documentation = [ "man:lxd(1)" ];
 
-      path = optional cfg.zfsSupport config.boot.zfs.package;
+      path = [ pkgs.util-linux ]
+        ++ optional cfg.zfsSupport config.boot.zfs.package;
 
       serviceConfig = {
         ExecStart = "@${cfg.package}/bin/lxd lxd --group lxd";