Function env_duration_ms

crates/ffi/src/kernel.rs:2093–2098

(var: &str, default: Duration)

    return LeanIOResult::error_string(
      "rs_env_extract: no constant names given",
    );
  }

  let bytes = match std::fs::read(&path) {
    Ok(bytes) => bytes,
    Err(e) => {
      return LeanIOResult::error_string(&format!(
        "rs_env_extract: failed to read {path}: {e}"
      ));