ci: Revisit use of actions-rs due to upstream deprecation

[tim-hm]

actions-rs is without a maintainer and so we will need to consider our options.

Ref: actions-rs/meta#43

Edit by Zoe:

Tasks:

• Find an alternative to rust-toolchain
• Find an alternative to clippy

[ZoopOTheGoop]

Alternatives that are maintained:
https://github.com/orgs/crusty-pie/repositories
https://github.com/dtolnay/rust-toolchain

We have also successfully replaced the clippy action already.