cargo clippy --all-targets already type-checks everything cargo check
would, so running both for each of the three feature/target
combinations was roughly doubling compile time in this job.
Switches from the docker/rust:1-bookworm container back to the rust-ci
runner label, and adopts sporah's build/test/audit job split with
cargo registry + sccache caching. Keeps feedsignal's workspace-specific
checks (native crates, web crate server/wasm-client features) and adds
clippy + cargo audit, which were previously missing.
