Fix Formula for fish-app 3.0.2

[sanssecours]

This commit fixes fish-shell/fish-shell#5703.

The command

brew cask style --fix fish-app

failed with the following error message on my computer:

Traceback (most recent call last):
	2: from /Users/rene/.rbenv/versions/2.6.1/bin/rubocop:23:in `<main>'
	1: from /Users/rene/.rbenv/versions/2.6.1/lib/ruby/2.6.0/rubygems.rb:302:in `activate_bin_path'
/Users/rene/.rbenv/versions/2.6.1/lib/ruby/2.6.0/rubygems.rb:283:in `find_spec_for_exe': can't find gem rubocop (>= 0.a) with executable rubocop (Gem::GemNotFoundException)

. Unfortunately installing rubocop via

gem install rubocop

did not fix the problem.

After making all changes to the cask:

• brew cask audit --download fish-app is error-free.
• brew cask style --fix fish-app reports no offenses.
• The commit message includes the cask’s name and version.
• The submission is for a stable version or documented exception.

[reitermarkus]

brew cask style should not be using Ruby from rbenv, not sure what's up with that.

[sanssecours]

brew cask style should not be using Ruby from rbenv, not sure what's up with that.

I would consider that normal behavior, since I changed the Ruby version via HOMEBREW_RUBY_PATH 😊. Unfortunately brew bump-formula-pr --strict does not work, if I use the vendor version of Ruby (see also: Homebrew/brew#5561).