[5.8] Allow retrieving env variables with getenv

[matt-allan]

This PR makes it possible to retrieve .env values with getenv, which is necessary for some third party libraries to work as expected.

This restores the 5.7 behavior. See https://github.com/vlucas/phpdotenv#upgrading-from-v2 for more info.

Resolves #27949 and #27913 .

[matt-allan]

I just saw #27828 (comment), which makes it sound like this change was intentional? I didn't think it was intentional as it wasn't documented in the upgrade guide.

@driesvints could you expand on that comment, I don't really understand what you mean. Won't the env helper still be read only as long as it isn't calling putenv or setting a key in the superglobals?

[ludo237]

I think the change was intentional but it also forces the whole PHP Packages ecosystem to adapt to Laravel which is crazy.

[driesvints]

@matt-allan please read #27949 (comment) and other remarks in the issue

[matt-allan]

Got it, thanks @driesvints. Looks like Taylor just committed a fix for LoadEnvironmentVariables but it's missing from helpers.php.

I just rebased this and fixed the conflict so it can be merged if we want to update helpers.php as well.

[staudenmeir]

@taylorotwell The current release breaks php artisan serve. We also have to adjust the env() helper.