nix_secupdates.py: initial version

[henrirosten]

nix_secupdates.py: initial version

• Add nix_secupdates.py script that demonstrates finding potential security updates for nix dependencies. See more details in the README.md section _nixsecupdates.
• Add repology_cve.py for querying CVEs that impact given package and version. This script is needed by nix_secupdates.py.
• Add support for querying also buildtime packages with nix_outdated.py.
• Import vulnxscan in shell.nix.
• Add more common functions to utils.py.
• Update relevant README.md.