tweak: Broken on Darwin

http://hydra.nixos.org/build/38316344

pkgs/applications/editors/tweak/default.nix

···
     description = "An efficient hex editor";
     homepage = "http://www.chiark.greenend.org.uk/~sgtatham/tweak";
     license = licenses.mit;
-    platforms = platforms.unix;
+    platforms = platforms.linux;
   };
 }