Optionally enforcing reproducility specifying which NVD archive to use

[aciceri]

It would be convenient having a CLI option to specify which NVD archive to use. Indeed I'm running vulnix in a CI pipeline and, after re-running it (same commit), I've realized that the state changed due to vulnix non reproducibility.

@ckauhaus I've seen that you are looking for a new maintainer so I don't know how much time you would spend on reviewing my potential PR, but if my proposal makes sense and you are interested let me know and I'll try working on a PR :)