fix: lightningcss-loader not work when using PostCSS

[chenjiahan]

Summary

When using Rspack's builtin:lightningcss-loader like this:

postcss-loader ---passing AST meta---> builtin:lightningcss-loader ---passing AST meta---> css-loader

The builtin:lightningcss-loader will not take any effect, because css-loader will read the AST meta from postcss-loader, and drop the transformation result of builtin:lightningcss-loader.

Related Links

web-infra-dev/rspack#7819
#3247

Checklist

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

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