ci: bump apify/actions/pnpm-install to v1.3.1 (#913)

Bumps the pinned `apify/actions/pnpm-install` composite action to
`v1.3.1`.

That release updates `actions/cache` to `v6.1.0`, so the action runs on
the Node.js 24 runtime. It clears the "Node.js 20 is deprecated" warning
that shows up on every CI run. For details, see apify/actions#29.