zsnes: only build on Linux

It uses callPackage_i686 which forces Linux anyway, and the current setup
just means that we end up with a weird i686-linux job under Darwin.

pkgs/misc/emulators/zsnes/default.nix

···
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = [ stdenv.lib.maintainers.sander ];
     homepage = http://www.zsnes.com;
-    platforms = stdenv.lib.platforms.unix;
+    platforms = stdenv.lib.platforms.linux;
   };
 }