Revert pub to script snapshots in the GN build. Fixes #28346.

utils/pub/BUILD.gn

 # Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 import("../application_snapshot.gni")
 
-application_snapshot("pub") {
+script_snapshot("pub") {
   main_dart = "../../third_party/pkg/pub/bin/pub.dart"
   training_args = [ "--help" ]
   deps = [
     "../compiler:dart2js_files_stamp",
   ]
   dart2js_gen_dir =
       get_label_info("../compiler:dart2js_files_stamp", "target_gen_dir")
   inputs = [
     "$dart2js_gen_dir/dart2js_files.stamp",
   ]
 }