CI / Security audit (RUSTSEC) (pull_request) Failing after 3s
CI / Build + Clippy + Test (pull_request) Failing after 44m45s
The catthehacker/ubuntu:act-latest image does not have rustup in PATH
('rustup: command not found', exit 127). Replace the direct rustup
invocation with the official install script from sh.rustup.rs, which
installs rustup + the stable toolchain in one step.
Also adds ~/.cargo/bin to GITHUB_PATH so subsequent steps (cargo clippy,
cargo build, cargo test, cargo audit) can find cargo/rustc.