CI: move away from actions-rs

[sadlerap]

As of October 13th, 2023, most of the actions in actions-rs have been archived. Even before then, some actions haven't seen an update in quite a long time (for instance, cargo hasn't received any updates since July 2021).

This might be a problem soon, since github is deprecating support for actions running on node12 and forcing them to use node16 runners. Since these are unmaintained, there's a possibility these actions could break without any possibility of fixes landing upstream.

It might be a good idea to find an alternative, though I'm not entirely sure what alternatives to these actions are available. Does the cargo that comes built into the ubuntu image runners support picking up the rust-toolchain file as a version override? If so, that might be an option.

[madsmtm]

There is https://github.com/dtolnay/rust-toolchain, which seems to be what a lot of people are gravitating towards