refactor: when ssg build, init only one rsbuild instance

[SoonIter]

Summary

make clientBuilder and serverBuilder merged into one rsbuild builder

some benefits

1. make DEBUG mode align with Rsbuild, and debug the ssg build easily
2. performance better (to be measured)
3. more maintainable

Related Issue

close #1167

Checklist

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

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[SoonIter]

pending with web-infra-dev/rsbuild#2620

Rspress would be the first show case of Rsbuild multi-environment support (SSG)

[SoonIter]

Rough estimate of performance

before

after