nethack: Fix platform logic

pkgs/games/nethack/default.nix

···
 
 let
   platform =
-    if lib.elem stdenv.system lib.platforms.unix then "unix"
+    if stdenv.hostPlatform.isUnix then "unix"
     else throw "Unknown platform for NetHack: ${stdenv.system}";
   unixHint =
-    if stdenv.isLinux then "linux"
-    else if stdenv.isDarwin then "macosx10.10"
+    /**/ if stdenv.hostPlatform.isLinux  then "linux"
+    else if stdenv.hostPlatform.isDarwin then "macosx10.10"
     # We probably want something different for Darwin
     else "unix";
   userDir = "~/.config/nethack";