test(addons): prewarm pnpm store before parallel installs in CI

[jycouet]

Closes #

Description

Experiment CI stuff: warm the pnpm store with the add-on test deps before the parallel installs, so test files hard-link from the store instead of each downloading cold.

Checklist

• Update snapshots (if applicable)
• Add a changeset (if applicable)
• Allow maintainers to edit this PR
• I care about what I'm doing, no matter the tool I use (Notepad, Sublime, VSCode, AI...)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 84da267

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[jycouet]

Not positive. Closing