* Update GitHub Actions
- actions-rs is unmaintained.
- See: https://github.com/actions-rs/toolchain/issues/216
- Enable rust-cache
- Cargo.lock was removed in #63 but it's necessary for reproducible builds in CI.
- The lock file is ignored by dependents.
- Our separate `pixels-ci-rust-version` repo also ignores it, so CI will still be able to tell us if the crate build ever breaks due to unlocked dependencies.
- See the `rust-version` job in ci.yml.