Update install.php

Use sys_get_temp_dir() to get the temp directory

core/install/install.php

@@ -357,7 +357,7 @@
 	$view = new template();
 	$view->engine = 'smarty';
 	$view->template_dir = $_SERVER["DOCUMENT_ROOT"].PROJECT_PATH.'/core/install/resources/views/';
-	$view->cache_dir = $_SESSION['server']['temp']['dir'];
+	$view->cache_dir = sys_get_temp_dir();
 	$view->init();
 
 //assign default values to the template