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

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:

.. code:: nix

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

will now output

.. code::

  127.0.0.1 localhost
  ::1 localhost

  127.0.0.1 myproject.localhost

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

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