Update workflows

[flodolo]

This PR includes changes from:

• ci: Use -C rather than -Z for instrument-coverage. #332: Use -C rathen than -Z for instrument-coverage
• ci: Stop using actions-rs/toolchain. #320: Replace unmaintained actions-rs/toolchain with dtolnay/rust-toolchain

It also updates actions/cache to v4.

[flodolo]

My goal here is to confirm that the workflows work, and have a log (the other two PRs are old enough that the logs have been deleted, and I couldn't figure out a way to re-run the actions without touching the forks).

Happy to merge those PRs to give credit where credit is due. The only addition here is updating actions/cache, because those deprecation warnings I could see in logs.

[flodolo]

Thanks Greg, I'll merge the other 2 PRs and rebase this one.