tmux: export ZSH_TMUX_CONFIG again to provide to tmux (#8390)

plugins/tmux/tmux.plugin.zsh

@@ -46,6 +46,7 @@ fi
 
 # Set the correct local config file to use.
 if [[ "$ZSH_TMUX_ITERM2" == "false" && -e "$ZSH_TMUX_CONFIG" ]]; then
+  export ZSH_TMUX_CONFIG
   export _ZSH_TMUX_FIXED_CONFIG="${0:h:a}/tmux.extra.conf"
 else
   export _ZSH_TMUX_FIXED_CONFIG="${0:h:a}/tmux.only.conf"