Revert "nixos/ddclient: fix permission for ddclient.conf (#148179)"

This reverts commit 6af3d13bec9b13e8fa8e19594ffbcbe085387bdd.

Reported by @arcnmx
(https://github.com/NixOS/nixpkgs/pull/148179#issuecomment-987197656):

  Does this not completely break the service? It doesn't change the
  owner to the same as the ddclient server (which is somewhat difficult
  due to it being a DynamicUser), so this now makes the service
  completely unusable because the config is only readable by its owner,
  root:

    ddclient[871397]: WARNING:  file /run/ddclient/ddclient.conf: Cannot open file '/run/ddclient/ddclient.conf'. (Permission denied)

  Given that the RuntimeDirectory was only readable by the ddclient
  service, the warning this PR fixes was spurious and not indicative of
  an actual information leak. I'm not sure of what a quick fix would be
  due to DynamicUser, but would at least request a revert of this so the
  service can work again?

nixos/modules/services/networking/ddclient.nix

@@ -29,7 +29,7 @@ let
   configFile = if (cfg.configFile != null) then cfg.configFile else configFile';
 
   preStart = ''
-    install --mode=0400 ${configFile} /run/${RuntimeDirectory}/ddclient.conf
+    install ${configFile} /run/${RuntimeDirectory}/ddclient.conf
     ${lib.optionalString (cfg.configFile == null) (if (cfg.passwordFile != null) then ''
       password=$(printf "%q" "$(head -n 1 "${cfg.passwordFile}")")
       sed -i "s|^password=$|password=$password|" /run/${RuntimeDirectory}/ddclient.conf