MAINT: 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Still using the outdated / unmaintained actions will generate several warnings in CI runs, for example in https://github.com/bluss/arrayvec/actions/runs/5299901385:

Node.js 12 actions are deprecated. Please update the following actions to use Node.js 16: actions/checkout@v2, actions-rs/toolchain@v1. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/.

The PR will get rid of those warnings.

[striezel]

@bluss: I just force pushed some changes, because meanwhile v4 of actions/checkout has been released, so the PR should use that newer version.