fix: Subsitemap pages are not stored in a custom cache driver (e.g. redis) when automatic chunking is enabled

[censujiang]

🐛 The bug

As the title suggests, only sitemap_index.xml is cached

🛠️ To reproduce

https://stackblitz.com/edit/nuxt-starter-s4npgl?file=nuxt.config.ts

🌈 Expected behavior

As long as I set up sitemap caching, all sitemap pages need to be cached to the correct drive, not to memory. If this is not the case, it will cause memory leaks when running on Bun.

ℹ️ Additional context

No response