kns: allow building on any unix platform

This package is fine to build on both Linux and macOS at the least.

pkgs/applications/networking/cluster/kns/default.nix

···
     homepage = "https://github.com/blendle/kns";
     license = licenses.isc;
     maintainers = with maintainers; [ mmlb ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }