fix: postcssrc cache may cause memory leak

[chenjiahan]

Summary

When running Rsbuild E2E tests, dozens of postcss configurations were stored in postcsssrcCache, which caused memory usage.

We can move postcssrcCache to the plugin scope to avoid memory leaks.

Checklist

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

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	138fee9
🔍 Latest deploy log	https://app.netlify.com/sites/rsbuild/deploys/67b6a6be9bc4f70008cf7ed9
😎 Deploy Preview	https://deploy-preview-4604--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 49 (🟢 up 1 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.