Is it possible to run Craft CMS with putenv() function disabled?
DotEnv only calls putenv if it sees the function is available since version 2.4.0, so make sure you're running at least that: https://github.com/vlucas/phpdotenv/commit/58633a061610ffd731d7fbbc8f7f009d14b2e4cf

Even then, I'm pretty sure other Craft dependencies (like Composer) will still require putenv support.