If a NULL xkb_rule_names pointer is passed to
xkb_keymap_new_from_names, libxkbcommon will default to reading
the XKB_* env variables. So there's no need to do it ourselves.
Also s/xkb_map_new_from_names/xkb_keymap_new_from_names/ since the
latter is more consistent with the returned struct name.
[1]: https://xkbcommon.org/doc/current/structxkb__rule__names.html