Allow specifying the environment for the kernel using an environment variables

There's https://symfony.com/components/Dotenv

This isn't about adding a package to load env files, but just allowing customizing the parameter without managing index.php yourself

Then it should use getenv() as the rest of testing code doing

A discussion in ✨ Making Drupal automatically installable with default environment variables Needs work went from getenv to $_ENV. I'm somewhat indifferent; whatever makes this land faster. There are benefits to each. But if Drupal core is already using getenv then I'll switch