Function run_cargo_build 

async fn run_cargo_build(
    config: &CargoBuilderConfig,
    target: Option<&str>,
    triple: &str,
    version: &str,
    abbaye_version: &str,
    line_tx: UnboundedSender<String>,
    target_dir: Option<&Path>,
) -> Result<Vec<ArtifactPath>>

Spawn cargo build --release --message-format=json [--target <triple>] [--manifest-path <path>] and collect every artifact path from the compiler-artifact messages.

Stderr lines are forwarded to line_tx as plain strings; the caller is responsible for mapping them to the appropriate LogEvent variant.