fix(cache): buildCache.cacheDirectory config not work

[Shucei]

Summary

Fix: Rspack cache directory config not taking effect due to incorrect config structure

When configuring performance.buildCache.cacheDirectory in Rsbuild, the cache directory setting was not being applied correctly for Rspack. This is because Rspack requires the cache directory to be configured under experiments.cache.storage.directory, while the current implementation was placing it directly under experiments.cache.directory.

Before:

chain.experiments({
  cache: {
    type: 'persistent',
    directory: cacheDirectory, // ❌ incorrect placement
    buildDependencies,
    version: cacheVersion
  }
});

## Related Links
Rspack cache configuration documentation: https://rspack.dev/config/cache

<!--- Provide links of related issues or pages -->

## Checklist
Updated the cache directory configuration structure in packages/core/src/plugins/cache.ts

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- [ ] Tests updated (or not required).
- [ ] Documentation updated (or not required).

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[chenjiahan]

Thank you!