Add nix support #201

 * There are two main functions: get_download_url (system‑specific binary) and get_src_download_url (source tarball).
 * Avoid using nix to retrieve data whenever possible, but provide a fallback to nix if the non‑Nix method does not yield results.

Signed-off-by: Chin Yeung Li <tli@nexb.com>

Author: chinyeungli