nspawn: check cg_ns_supported() just once

nspawn: check cg_ns_supported() just once

cg_ns_supported() caches, so the condition was really checked just once, but
it looks weird to assign the return value to arg_use_cgns (if the variable is not present),
because then the other checks are effectively equivalent to
  if (cg_ns_supported() && cg_ns_supported()) { ...
and later
  if (!cg_ns_supported() || !cg_ns_supported()) { ...