update GitHub Actions CI

[striezel]

The following updates are performed:

• update actions/checkout to v4
• replace unmaintained actions-rs/toolchain by dtolnay/rust-toolchain

Still using the outdated / unmaintained actions will generate several warnings in CI runs, for example in https://github.com/tafia/calamine/actions/runs/6153848349:

The following actions use node12 which is deprecated and will be forced to run on node16: actions/checkout@v2, actions-rs/toolchain@v1. For more info: https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

The PR will get rid of those warnings.

[tafia]

Thanks!