Replace incorrect variable (#6017)

Author: Thom

install/lib/installer_base.lib.php

@@ -2637,7 +2637,7 @@ public function configure_apps_vhost() {
 			$content = rfsel($conf['ispconfig_install_dir'].'/server/conf-custom/install/nginx_apps.vhost.master', 'tpl/nginx_apps.vhost.master');
 
 			// Enable SSL if a cert is in place.
-			if(is_file($conf['ispconfig_install_dir'].'/interface/ssl/ispserver.crt') && is_file($install_dir.'/interface/ssl/ispserver.key')) {
+			if(is_file($conf['ispconfig_install_dir'].'/interface/ssl/ispserver.crt') && is_file($conf['ispconfig_install_dir'].'/interface/ssl/ispserver.key')) {
 				$content = str_replace('{ssl_on}', 'ssl', $content);
 				$content = str_replace('{ssl_comment}', '', $content);
 			} else {