Change aspell configure to use $out/lib/aspell

This change is need so that the following comment will work:

```
  # Note: Users should define the `ASPELL_CONF' environment variable to
  # `dict-dir $HOME/.nix-profile/lib/aspell/' so that they can access
  # dictionaries installed in their profile.
```

And now I have Nix's `aspell` driving spelling in my Emacs!