ci: remove redundant pnpm installation from ecosystem ci

[chenjiahan]

Summary

Remove redundant pnpm installation from ecosystem ci as these steps were redundant.

Checklist

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

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[Copilot]

Pull Request Overview

This PR removes redundant pnpm installation steps from the ecosystem CI workflow. The changes eliminate manual corepack installation and enabling commands that were duplicated across multiple jobs, streamlining the CI configuration.

Key changes:

• Removed "Install Pnpm" steps that manually installed and enabled corepack in two separate jobs
• Removed pnpm cache configuration from the Node.js setup action

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