Update ignored directories in gh-pages step (#2150)

Follow up to https://github.com/flutter/samples/pull/2144

cc: @kevmoo

web/_tool/build_ci.dart

@@ -7,7 +7,7 @@ import 'package:path/path.dart' as p;
 import 'common.dart';
 import 'fix_base_tags.dart';
 
-const ignoredDirectories = ['_tool', 'samples_index'];
+const ignoredDirectories = ['_tool', '_packages', 'samples_index'];
 
 void main() async {
   final packageDirs = [