ci: remove pnpm cache

[chenjiahan]

Summary

Motivation:

• We recently encountered some issues with cache causing the Windows unit testing CI to fail (https://github.com/web-infra-dev/rsbuild/actions/runs/18751006482/job/53490332365)
• After comparison, enabling on the cache did not make Rsbuild CI run faster

From https://pnpm.io/continuous-integration:

In all the provided configuration files the store is cached. However, this is not required, and it is not guaranteed that caching the store will make installation faster. So feel free to not cache the pnpm store in your job.

Checklist

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

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[Copilot]

Pull Request Overview

This PR removes the pnpm cache configuration from the GitHub Actions test workflow. This appears to be an experimental change to test workflow behavior without caching.

Key Changes:

• Removed the cache: 'pnpm' parameter from the actions/setup-node step in the test workflow

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