如果 gem 早期加载 ActiveJob::Arguments,则 8.1.3 中的自定义活动作业序列化器仍会被忽略

This is a follow-up to #56090, which was fixed by #56093 in 8.1.2. That fix covers the case where ActiveJob::Arguments has not been loaded yet. The opposite case is still broken: if anything loads ActiveJob::Arguments before the application's initializers run, a serializer registered the way the guide prescribes is never registered at all.