tarball: fix evaluation of mdadm on darwin

It probably doesn't make any sense to have mdadm on darwin,
but we have to "fail in a right way".

pkgs/os-specific/linux/mdadm/default.nix

···
 { stdenv, fetchurl, groff }:
 
+assert stdenv.isLinux;
+
 stdenv.mkDerivation rec {
   name = "mdadm-3.3.4";