refactor: simplify build dependencies merging

[chenjiahan]

Summary

Updates the build dependency handling logic in the caching plugin, simplifying how additional build dependencies are passed and stored.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR refactors the build dependencies merging logic in the caching plugin, changing from a flexible multi-group approach to a simpler single-array approach. The refactoring reduces complexity but also changes the API surface.

Key Changes:

• Changed getBuildDependencies parameter from userBuildDependencies: Record<string, string[]> to additionalDependencies?: string[]
• Replaced object spreading with direct property assignment (buildDependencies.additional = additionalDependencies)
• Simplified the call site by passing cacheConfig.buildDependencies directly instead of wrapping it in an object

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