cc-3473: use euid in airtime-check-system

utils/airtime-check-system.php

@@ -32,8 +32,9 @@ class AirtimeCheck {
     public static function ExitIfNotRoot()
     {
         // Need to check that we are superuser before running this.
+        $euid = posix_geteuid();
         $user = posix_getlogin();
-        if($user != "root" && $user != "www-data"){
+        if($euid != 0 && $user != "www-data"){
             echo "Must be root user.\n";
             exit(1);
         }