ci: use pnpm/action-setup on Windows for e2e tests

[serhalp]

Summary

Corepack on Windows wasn't properly respecting the npm_config_registry environment variable when running pnpm commands, causing E2E tests to fail with "No matching version found for netlify-cli@testing" because pnpm was fetching from npmjs.org instead of the local Verdaccio server.

The e2e tests for pnpm were implicitly using whatever happened to be the latest pnpm. pnpm@11 was recently released and broke some assumption somewhere.

This removes the weird Windows exception.

[coderabbitai]

📝 Walkthrough

Walkthrough

The E2E workflow consolidates pnpm package manager setup by replacing prior conditional Windows/non-Windows initialization steps with a single unified "Setup pnpm" step, while retaining the downstream pnpm/action-setup@v4 action configured for pnpm version 10.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: switching to pnpm/action-setup on Windows for e2e tests to fix registry configuration issues.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description clearly explains the problem (Corepack not respecting npm_config_registry on Windows), the impact (E2E test failures), and the solution (removing Windows exception and using pnpm/action-setup consistently).

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/slack-session-iYgke

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[github-actions]

📊 Benchmark results

Comparing with 0a70cf4

• Dependency count: 1,129 (no change)
• Package size: 379 MB (no change)
• Number of ts-expect-error directives: 355 (no change)