(tmux) Add KITTY_REMOTE_ON to list of variables to copy to session

-g = global session or window option
-a = append
update-environment[] variable
             Set list of environment variables to be copied into the
             session environment when a new session is created or an
             existing session is attached.  Any variables that do not
             exist in the source environment are set to be removed from
             the session environment (as if -r was given to the
             set-environment command).

.tmux.conf

@@ -8,6 +8,7 @@ set-option -g status-left "|"
 set-option -g status-right "" 
 set-option -g window-status-format "#{window_index} - #{window_name} |"
 set-option -g window-status-current-format "* #{window_index} - #{window_name} |"
+set-option -ga update-environment "KITTY_LISTEN_ON"
 
 set-option -g @plugin "arcticicestudio/nord-tmux"
 set-option -g @plugin "christoomey/vim-tmux-navigator"