Revert sync-workspace pnpm install workaround

[richardhjtan]

Summary

• Reverts the prior workaround commit that switched the sync action from npm install -g @cardstack/boxel-cli to npm install -g pnpm && pnpm add -g @cardstack/boxel-cli.
• The workaround does not work: pnpm refuses to install the published boxel-cli with ERR_PNPM_SPEC_NOT_SUPPORTED_BY_ANY_RESOLVER because @cardstack/boxel-cli@0.1.0's tarball contains raw catalog: specifiers ("catalog: only resolves inside the publishing workspace").
• npm fails with EUNSUPPORTEDPROTOCOL for the same reason.

The real fix lives in the monorepo — packages/boxel-cli must be published with pnpm publish instead of npm publish so catalog: deps get rewritten to concrete versions in the tarball. PR for that: cardstack/boxel#fix-boxel-cli-use-pnpm-publish.

Until that lands and a new boxel-cli is published, the sync action is broken regardless of which installer it uses — reverting keeps the workflow file matching the historical baseline rather than carrying a workaround that doesn't help.

Test plan

• CI workflow loads sync-to-workspace.yml without syntax errors.
• No verification needed for the sync itself: it will continue to fail on npm install -g @cardstack/boxel-cli until the monorepo PR + republish are done.

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