ci.yml: use actions/checkout@v4

ci.yml: use actions/checkout@v4

This addresses the following warning from GitHub Actions:

    Node.js 16 actions are deprecated. Please update the following
    actions to use Node.js 20: actions/checkout@v3. For more information
    see:
    https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

A near-identical warning forced everyone to upgrade to v3 last year, so
this is some pointless churn, but let's just get it over with again...