formula_info: use RUBY_PATH instead of calling HOMEBREW_BREW_FILE
formula_info: use RUBY_PATH instead of calling HOMEBREW_BREW_FILE

This ensures correct RUBY_PATH is used when running `brew tests` with
custom `HOMEBREW_RUBY_PATH`

More info: https://github.com/Homebrew/brew/pull/6617#discussion_r336431131

claui/brew,EricFromCanada/brew,reitermarkus/brew,sjackman/homebrew,sjackman/homebrew,Homebrew/brew,reitermarkus/brew,JCount/brew,reitermarkus/brew,sjackman/homebrew,mahori/brew,vitorgalvao/brew,EricFromCanada/brew,sjackman/homebrew,vitorgalvao/brew,nandub/brew,vitorgalvao/brew,konqui/brew,MikeMcQuaid/brew,Homebrew/brew,JCount/brew,nandub/brew,mahori/brew,konqui/brew,vitorgalvao/brew,EricFromCanada/brew,claui/brew,claui/brew,konqui/brew,mahori/brew,nandub/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,MikeMcQuaid/brew,mahori/brew,reitermarkus/brew,MikeMcQuaid/brew,EricFromCanada/brew,Homebrew/brew,claui/brew,JCount/brew,JCount/brew,konqui/brew