fzf-tmux: Fix escaping of environment variables

[laloch]

This fixes fzf-tmux failing if FZF_DEFAULT_OPTS or FZF_DEFAULT_COMMAND contain multiple arguments. Example of failing setup:

$ export FZF_DEFAULT_OPTS="--tiebreak=index --ansi --border"
$ export FZF_DEFAULT_COMMAND="fd --color always"
$ fzf-tmux

This also adds PATH to the fzf's environment to help it find the fd executable in the above example.

[junegunn]

I can't reproduce the problem, the example works just fine.

$ export FZF_DEFAULT_OPTS="--tiebreak=index --ansi --border"
$ export FZF_DEFAULT_COMMAND="fd --color always"
$ fzf-tmux