nc4nix: allow Darwin builds

`nc4nix` builds and runs on Darwin, so add it to the platforms list.

pkgs/by-name/nc/nc4nix/package.nix

···
     homepage = "https://github.com/helsinki-systems/nc4nix";
     license = lib.licenses.mit;
     maintainers = with lib.maintainers; [ onny ];
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.unix;
   };
 }