nixos/weechat: Use parameter instead of env variable for state dir

The environment variable hides the actual state directory of weechat in
systemctl. To make it more obvious, use the equivalent CLI parameter.

This doesn't change the current behaviour.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

nixos/modules/services/misc/weechat.nix

@@ -45,13 +45,12 @@ in
     };
 
     systemd.services.weechat = {
-      environment.WEECHAT_HOME = cfg.root;
       serviceConfig = {
         User = "weechat";
         Group = "weechat";
         RemainAfterExit = "yes";
       };
-      script = "exec ${config.security.wrapperDir}/screen -Dm -S ${cfg.sessionName} ${cfg.binary}";
+      script = "exec ${config.security.wrapperDir}/screen -Dm -S ${cfg.sessionName} ${cfg.binary} --dir ${cfg.root}";
       wantedBy = [ "multi-user.target" ];
       wants = [ "network.target" ];
     };