You can create your own helpers.php file and override the getenv() function there. As long as you load your helpers file first, then Laravel will never register it's own, since all of the helpers are wrapped with if ( ! function_exists('function_name')).
See this thread for some details on overriding functions in Laravel's helpers.php file.
https://laracasts.com/discuss/channels/general-discussion/override-functions-in-supporthelpersphp