- Update action versions.
- Stop running CI on Mac OS entirely.
- Use the Rust version that comes with CI instead of installing stable.
(vendor in the GitHub annotations matcher from `setup-rust-toolchain`)
- Only run build and tests at the CI runner's (stable) Rust version.
- Only use the MSRV version for the "check minimal versions" job.
- Remove the "determine MSRV" job.
- For Clippy, use the same nightly version as was used for the cache.
- Only update the caches every week.
