Fix ISO building from a channel

Building config.system.build.isoImage would fail with the following
error using the channel:

  ln: failed to create symbolic link
'/nix/store/zz0hzi5imrg4927v6f8mv281qs6v6pbq-nixos-16.03pre69762.e9162732/nixos/nixpkgs/.': File exists

The fix skips symlink as it already exists if the channel
nixpkgs copy is used.

Fixes #10367