nixos/ddclient: fix missing 'password=' config key

The `password=` prefix was missing for the `protocol == nsupdate` and
`secretsFile != null` cases. As a result, there's a line in the
generated config file containing only the path to the secrets file / the
`secrets_placeholder`, which `ddclient` interprets as a hostname for
which to update a DNS entry, which won't work for obvious reasons.

nixos/modules/services/networking/ddclient.nix

@@ -20,11 +20,11 @@ let
     ${lib.optionalString (cfg.username != "") "login=${cfg.username}"}
     ${
       if cfg.protocol == "nsupdate" then
-        "/run/${RuntimeDirectory}/ddclient.key"
+        "password=/run/${RuntimeDirectory}/ddclient.key"
       else if (cfg.passwordFile != null) then
         "password=@password_placeholder@"
       else if (cfg.secretsFile != null) then
-        "@secrets_placeholder@"
+        "password=@secrets_placeholder@"
       else
         ""
     }