fix: use recommended way of updating Fish shell $PATH

fixes #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.

Library/Homebrew/test/utils/shell_spec.rb

@@ -98,7 +98,7 @@
       ENV["SHELL"] = "/usr/local/bin/fish"
       ENV["fish_user_paths"] = "/some/path"
       expect(described_class.prepend_path_in_profile(path))
-        .to eq("echo 'set -g fish_user_paths \"#{path}\" $fish_user_paths' >> #{shell_profile}")
+        .to eq("echo 'fish_add_path #{path}' >> #{shell_profile}")
     end
   end
 end

Library/Homebrew/utils/shell.rb

@@ -78,7 +78,7 @@ def prepend_path_in_profile(path)
       when :csh, :tcsh
         "echo 'setenv PATH #{csh_quote(path)}:$PATH' >> #{profile}"
       when :fish
-        "echo 'set -g fish_user_paths \"#{sh_quote(path)}\" $fish_user_paths' >> #{profile}"
+        "echo 'fish_add_path #{sh_quote(path)}' >> #{profile}"
       end
     end