softether module: fix dataDir option

nixos/modules/services/networking/softether.nix

···
 let
   cfg = config.services.softether;
 
+  package = cfg.package.override { dataDir = cfg.dataDir; };
+
 in
 {
 
···
 
       dataDir = mkOption {
         type = types.string;
-        default = "${cfg.package.dataDir}";
+        default = "/var/lib/softether";
         description = ''
           Data directory for SoftEther VPN.
         '';
···
   config = mkIf cfg.enable (
 
     mkMerge [{
-      environment.systemPackages = [
-          (pkgs.lib.overrideDerivation cfg.package (attrs: {
-            dataDir = cfg.dataDir;
-          }))
-        ];
+      environment.systemPackages = [ package ];
+
       systemd.services."softether-init" = {
         description = "SoftEther VPN services initial task";
         wantedBy = [ "network.target" ];
···
             for d in vpnserver vpnbridge vpnclient vpncmd; do
                 if ! test -e ${cfg.dataDir}/$d; then
                     ${pkgs.coreutils}/bin/mkdir -m0700 -p ${cfg.dataDir}/$d
-                    install -m0600 ${cfg.package}${cfg.dataDir}/$d/hamcore.se2 ${cfg.dataDir}/$d/hamcore.se2
+                    install -m0600 ${package}${cfg.dataDir}/$d/hamcore.se2 ${cfg.dataDir}/$d/hamcore.se2
                 fi
             done
             rm -rf ${cfg.dataDir}/vpncmd/vpncmd
-            ln -s ${cfg.package}${cfg.dataDir}/vpncmd/vpncmd ${cfg.dataDir}/vpncmd/vpncmd
+            ln -s ${package}${cfg.dataDir}/vpncmd/vpncmd ${cfg.dataDir}/vpncmd/vpncmd
         '';
       };
     }
···
         wantedBy = [ "network.target" ];
         serviceConfig = {
           Type = "forking";
-          ExecStart = "${cfg.package}/bin/vpnserver start";
-          ExecStop = "${cfg.package}/bin/vpnserver stop";
+          ExecStart = "${package}/bin/vpnserver start";
+          ExecStop = "${package}/bin/vpnserver stop";
         };
         preStart = ''
             rm -rf ${cfg.dataDir}/vpnserver/vpnserver
-            ln -s ${cfg.package}${cfg.dataDir}/vpnserver/vpnserver ${cfg.dataDir}/vpnserver/vpnserver
+            ln -s ${package}${cfg.dataDir}/vpnserver/vpnserver ${cfg.dataDir}/vpnserver/vpnserver
         '';
         postStop = ''
             rm -rf ${cfg.dataDir}/vpnserver/vpnserver
···
         wantedBy = [ "network.target" ];
         serviceConfig = {
           Type = "forking";
-          ExecStart = "${cfg.package}/bin/vpnbridge start";
-          ExecStop = "${cfg.package}/bin/vpnbridge stop";
+          ExecStart = "${package}/bin/vpnbridge start";
+          ExecStop = "${package}/bin/vpnbridge stop";
         };
         preStart = ''
             rm -rf ${cfg.dataDir}/vpnbridge/vpnbridge
-            ln -s ${cfg.package}${cfg.dataDir}/vpnbridge/vpnbridge ${cfg.dataDir}/vpnbridge/vpnbridge
+            ln -s ${package}${cfg.dataDir}/vpnbridge/vpnbridge ${cfg.dataDir}/vpnbridge/vpnbridge
         '';
         postStop = ''
             rm -rf ${cfg.dataDir}/vpnbridge/vpnbridge
···
         wantedBy = [ "network.target" ];
         serviceConfig = {
           Type = "forking";
-          ExecStart = "${cfg.package}/bin/vpnclient start";
-          ExecStop = "${cfg.package}/bin/vpnclient stop";
+          ExecStart = "${package}/bin/vpnclient start";
+          ExecStop = "${package}/bin/vpnclient stop";
         };
         preStart = ''
             rm -rf ${cfg.dataDir}/vpnclient/vpnclient
-            ln -s ${cfg.package}${cfg.dataDir}/vpnclient/vpnclient ${cfg.dataDir}/vpnclient/vpnclient
+            ln -s ${package}${cfg.dataDir}/vpnclient/vpnclient ${cfg.dataDir}/vpnclient/vpnclient
         '';
         postStart = ''
             sleep 1

pkgs/servers/softether/4.18.nix

···
 { stdenv, fetchurl
-, openssl, readline, ncurses, zlib }:
+, openssl, readline, ncurses, zlib
+, dataDir ? "/var/lib/softether" }:
 
 let
   os = if stdenv.isLinux then "1"
···
   version = "4.18";
   build = "9570";
   compiledDate = "2015.07.26";
-  dataDir = "/var/lib/softether";
 
   src = fetchurl {
     url = "http://www.softether-download.com/files/softether/v${version}-${build}-rtm-${compiledDate}-tree/Source_Code/softether-src-v${version}-${build}-rtm.tar.gz";

pkgs/servers/softether/4.20.nix

···
 { stdenv, fetchurl
-, openssl, readline, ncurses, zlib }:
+, openssl, readline, ncurses, zlib
+, dataDir ? "/var/lib/softether" }:
 
 let
   os = if stdenv.isLinux then "1"
···
   version = "4.20";
   build = "9608";
   compiledDate = "2016.04.17";
-  dataDir = "/var/lib/softether";
 
   src = fetchurl {
     url = "http://www.softether-download.com/files/softether/v${version}-${build}-rtm-${compiledDate}-tree/Source_Code/softether-src-v${version}-${build}-rtm.tar.gz";

pkgs/servers/softether/4.25.nix

···
 { stdenv, fetchurl
-, openssl, readline, ncurses, zlib }:
+, openssl, readline, ncurses, zlib
+, dataDir ? "/var/lib/softether" }:
 
 let
   os = if stdenv.isLinux then "1"
···
   version = "4.25";
   build = "9656";
   compiledDate = "2018.01.15";
-  dataDir = "/var/lib/softether";
 
   src = fetchurl {
     url = "http://www.softether-download.com/files/softether/v${version}-${build}-rtm-${compiledDate}-tree/Source_Code/softether-src-v${version}-${build}-rtm.tar.gz";