fix: pin @rsbuild/core version to avoid @rspack/core 1.5.5

[fi3ework]

Summary

use ~ until web-infra-dev/rspack#11630 landed.

Related Links

Checklist

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

[Copilot]

Pull Request Overview

This PR pins the @rsbuild/core dependency to a specific version to avoid compatibility issues with @rspack/core 1.5.5.

• Removes tilde range specifier from @rsbuild/core dependency
• Pins version to exactly 1.5.4 to prevent automatic updates

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