[#63] Replace $SHLVL with $TMUX

plugins/tmux/init.zsh

@@ -20,9 +20,7 @@ alias ta="tmux attach-session"
 alias tl="tmux list-sessions"
 
 # Auto Start
-if (( $SHLVL == 1 )) && zstyle -t ':omz:plugin:tmux:auto' start; then
+if [[ -z "$TMUX" ]] && zstyle -t ':omz:plugin:tmux:auto' start; then
-  (( SHLVL += 1 )) && export SHLVL
-
   session="$(
     tmux list-sessions 2> /dev/null \
       | cut -d':' -f1 \