ci: update actions/checkout in GitHub Actions workflows to v4

[striezel]

Updates the actions/checkout action used in the GitHub Actions workflows to its newest major version.

Still using v3 of actions/checkout will generate some warning like in this run: https://github.com/FourierTransformer/lua-simdjson/actions/runs/10173326514

The following actions use a deprecated Node.js version and will be forced to run on node20: actions/checkout@v3. For more info: https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

The PR will get rid of those warnings for actions/checkout, because v4 uses Node.js 20.