nix flake: use `lib.mkAfter` put the host list at the end

At present, if a user uses networking.stevenBlockHosts.enable = true in tandem with networking.extraHosts, the extraHosts will put be put after which makes it very difficult to see the custom adds with even the $PAGER operating a bit slow due to file size. I would propose putting this project’s hosts at the end of the hosts file. Meaning:

{
  networking = {
    stevenBlockHosts.enable = true;
    extraHosts = ''
      127.0.0.1 myproject.localhost
    '';
  };
}

will now output

127.0.0.1 localhost
::1 localhost

127.0.0.1 myproject.localhost

# Title: StevenBlack/hosts with the fakenews extension
#
# …

NOTE: using lib.* like !2 to avoid a potential merge conflict