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
  #
  # …

Format: text/x-rst

flake.nix

@@ -38,7 +38,7 @@
                 orig = builtins.readFile ("${self}/" + (lib.optionalString (alternatesList != []) alternatesPath) + "hosts");
                 ipv6 = builtins.replaceStrings [ "0.0.0.0" ] [ "::" ] orig;
               in
-              orig + (lib.optionalString cfg.enableIPv6 ("\n" + ipv6));
+              lib.mkAfter (orig + (lib.optionalString cfg.enableIPv6 ("\n" + ipv6)));
           };
         };