fix: define NODE_ENV conflicts when setting mode

[chenjiahan]

Summary

Fix define NODE_ENV conflicts when setting mode, Rspack already defines process.env.NODE_ENV by Rspack's mode, so Rsbuild do not need to define it again.

Related Links

https://rspack.dev/config/mode

Checklist

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

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