fix: setup command cannot update `Config\Autoload::$helpers` with multiple lines

[kenjis]

Description See https://forum.codeigniter.com/showthread.php?tid=90747&pid=418193#pid418193

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [x] Conforms to style guide