Update install instructions for fish shell

[mtjn]

I couldn't open a pull request because I couldn't find the file to edit for this page: https://yarnpkg.com/en/docs/install#debian-stable

The recommended technique for setting PATH in fish is to use fish_user_paths like so:

set -U fish_user_paths (yarn global bin) $fish_user_paths

c.f. https://fishshell.com/docs/current/tutorial.html#tut_path and https://wiki.archlinux.org/index.php/Fish#Setting_fish_as_default_shell

Thanks!

[Haroenv]

the source is here https://github.com/yarnpkg/website/blob/master/lang/en/docs/_installations/unix_path_setup.md