Caching ~/.cargo/bin/ via actions/cache@v4 was poisoning the cargo
shim on macOS runners — restored cargo resolved to rustup-init and
failed with "unexpected argument 'build' found". Swatinem/rust-cache
skips that directory and handles target/ pruning + key derivation.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>