ci: use caching actions to setup build environment

[JarvisCraft]

Description

This enables the usage of GitHub Actions which cache dependencies by default in order to speed up builds.

[JarvisCraft]

@dtolnay's action seems to not perform caching by itself and @actions-rs are unmaintained. Probably manual caching should be done for the former.

[dcoles]

It looks like running apt install isn't too bad in our existing GitHub actions (I've seen it range from 4s to 16s).

Though it definately seems a good idea to use @dtolnay's toolchain action.

[str4d]

83 now relies on rust-toolchain.toml as the baseline source of truth, and uses the toolchain action where needed (to switch to e.g. beta).

[JarvisCraft]

Superseded by #83