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

Changed files
+2 -1
nixos
modules
virtualisation
+2 -1
nixos/modules/virtualisation/lxd.nix
···
];
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";