Laravel set the environment variable from controller

I'm trying to set the environment variable from controller but it throws an "Call to undefined method Dotenv\Dotenv::setEnvironmentVariable()"

    $env = new Dotenv(app()->environmentPath(), app()->environmentFile());
    $env->setEnvironmentVariable('APP_ENV_TEST', 'testing');

Laravel Version: 5.6

vlucas/phpdotenv: 2.4

via Chebli Mohamed