nixos: use nativeBuildInputs in make- iso9660-image and system-tarball

The tools used to create iso9660 images and tarballs are independent of
the platform of the closure contained within.

nixos/lib/make-iso9660-image.nix

···
 stdenv.mkDerivation {
   name = isoName;
   builder = ./make-iso9660-image.sh;
-  buildInputs = [ xorriso syslinux zstd libossp_uuid ];
+  nativeBuildInputs = [ xorriso syslinux zstd libossp_uuid ];
 
   inherit isoName bootable bootImage compressImage volumeID efiBootImage efiBootable isohybridMbrImage usbBootable;
 

nixos/lib/make-system-tarball.nix

···
 stdenv.mkDerivation {
   name = "tarball";
   builder = ./make-system-tarball.sh;
-  buildInputs = extraInputs;
+  nativeBuildInputs = extraInputs;
 
   inherit fileName extraArgs extraCommands compressCommand;