Simplify pnpm caching to auto-enable on GitHub-hosted runners
Remove the explicit enablePnpmCache option and instead automatically
enable pnpm store caching when runs_on_labels contains 'ubuntu-latest'.
This way any job using build_reusable.yml on GitHub-hosted runners
will benefit from pnpm caching without needing to remember to enable it.
Co-authored-by: sebastian.silbermann <sebastian.silbermann@vercel.com>
