fixes https://github.com/Homebrew/brew/issues/8965
See Fish docs https://fishshell.com/docs/current/tutorial.html#path.
> A faster way is to use the `fish_add_path function`... The advantage is that
> you don't have to go mucking around in files: just run this once at the
> command line, and it will affect the current session and all future instances
> too.
>
> Or you can modify `$fish_user_paths` yourself, but you should be careful not
> to append to it unconditionally in config.fish, or it will grow longer and
> longer.
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