Update CodeIgniter.php

Use getenv instead of $_SESSION in detectEnvironment

Author: fcosrno

system/CodeIgniter.php

@@ -528,7 +528,7 @@ protected function detectEnvironment()
     {
         // Make sure ENVIRONMENT isn't already set by other means.
         if (! defined('ENVIRONMENT')) {
-            define('ENVIRONMENT', $_SERVER['CI_ENVIRONMENT'] ?? 'production');
+            define('ENVIRONMENT', getenv('CI_ENVIRONMENT') ?? 'production');
         }
     }