ci: cache pnpm store with corepack

[b3hr4d]

Summary

• add explicit pnpm store caching back to GitHub Actions
• keep the Corepack-based pnpm setup that fixed the earlier CI failures
• avoid reintroducing actions/setup-node pnpm cache integration

Details

• activate pnpm with Corepack first
• resolve the pnpm store path dynamically via pnpm store path
• cache the pnpm store with actions/cache@v5
• key the cache by runner OS and pnpm-lock.yaml

Why

The previous setup-node pnpm cache integration conflicted with the new Corepack-based setup. This restores install performance without coupling cache restore to setup-node.