Update deprecated Actions

Several Actions or underlying commands are deprecated:

- everything under `actions-rs` is now deprecated ([`dtolnay/rust-toolchain`](https://github.com/dtolnay/rust-toolchain) seems to be the de facto replacement)
- `set-output` should be updated as per [this](https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/)