Clear stale cache artifacts built against old Rust versions

[echeran]

Prompted from build issues in PR #220 - @zbraniecki

Initial idea: if the compatibility of cached build artifacts to the current build also depend on Rust versions matching, then let's add the Rust version to the cache key.

To test: does running the rustc command on the Github Actions runner VMs automatically access the latest stable Rust, or do we need to run a rust command (rustup via actions-rs/toolchain?) to get the latest Rust?

[echeran]

What's the best way to get the Rust version? Run rustc --version and parse the output?

[zbraniecki]

I think so? https://docs.rs/semver/0.10.0/semver/

There's also https://github.com/dtolnay/rustversion

[sffc]

How much time does the cache actually save for us?

I think it's useful to cache things that require a download, but for compile artifacts, maybe we should just not perform caching since it seems to cause a number of problems.

[echeran]

Good question. Previously, caching turned a 4-5 min build into 0.5-1.5 min (see #102 (comment)).

But we've already had issues with macOS builds (actions-rs/cargo#111) and turned caching off there (#210), and we've added formatting and linting checks to the build & test workflow, and we're changed the testing step to run over all combinations of features (#172), so the speedup will be less noticeable now.

So I'm inclined to turn off caching and revisit later.

[zbraniecki]

I'm ok with trying to skip caching and see what happens.

But we should also consider (maybe later?) caching based on Rust version as per https://stackoverflow.com/questions/63521430/clear-cache-in-github-actions