[6.x] Fixed code that depended on getenv

[GrahamCampbell]

If the env variables are loaded without using putenv, then getenv won't be able to see them, so the current code is wrong. It should use Laravel's env helper.

[antonkomarev]

Maybe use Env support class instead of env helper method?

[GrahamCampbell]

Maybe use Env support class instead of env helper method?

I'll do that in src, but not in tests, since all of the other tests use the global helper, and none of them use the Env class directly.