[Break Change] Use $TMUX_FZF_LAUNCH_KEY

[sainnhe]

So you've noticed that this plugin uses @tmux-fzf-launch-key to customize the launch key, but uses environment variables to control some other behaviors.

That's because the performance of tmux variables is VERY BAD. I pushed a new branch named tmux-options to demonstrate how bad the performance will be if we use tmux variables. You can checkout this branch and get it a try.

I decided to switch from @tmux-fzf-launch-key to $TMUX_FZF_LAUNCH_KEY partly because of this, another reason is that I don't want to use tmux variables in some places while use environment variables in others, that makes the code and .tmux.conf very dirty.

You need to modify your config if you are using @tmux-fzf-launch-key after https://github.com/sainnhe/tmux-fzf/commit/66817c12b8d87284a891f818c58629e565b7eceb.