CI: add clippy and update toolchain install action.

[ThomasFrans]

• Update all the actions to the latest version since some of them were using deprecated stuff.
• Use a different Rust toolchain installation action because the previous one is no longer maintained and generated deprecated warnings. The new one is by dtolnay (the author of the syn, anyhow and cargo-expand) so it will probably be maintained for a while 🙂
• Fix the last clippy warning that occurred by enabling all features.
• Cache the build artifacts as long as the dependencies don't change.
• Build the project instead of just checking it (I'm not sure but I think just checking can miss errors under some circumstances. Some projects check, others build.). Building doesn't add much extra time when caching the target directory anyways.

I hope this is ok. This is only my second GitHub CI config I've worked on so if anything isn't right, I'll happily change it 🙂

The first run without any caches took around 10m for me, subsequent runs take around 3m40s as long as the Cargo.lock file doesn't change (which invalidates the dependency artifact caches).

Also it might be necessary to manually delete the latest caches (the ones that were used in the last while) as I don't think they will be updated with the target directory otherwise...

closes #1051

[hrkfdn]

Great! Thanks for tackling this. I have some minor comments below.

[ThomasFrans]

I'll update the things you mentioned (or you can do it if you prefer).

[hrkfdn]

Merged, thank you! :)

[ThomasFrans]

Merged, thank you! :)

I think you might have to clear out the current job cache once (the ones used in the last while, as I don't remember if there is a clear all button). I had some problems where the clippy and build files were using the same cache tag which isn't correct.

[hrkfdn]

Thanks for the heads up, I'll update the Cargo.lock via a dependency update which should change the cache key, right?