Nix integration

I'd love to be able to run an audit inside a `nix build`. It would be amazing if the `cabal-audit` flake could expose a `lib` function that turns an advisory repository, an "ignore" config file, and haskell packages into an audit check. Currently this isn't possible because: 1. There is no way to ignore advice 2. The `--file-patch` and `--repository` flags expect a directory with a `.git` directory and nix filters those out. 3. ... ? Who knows, I didn't get this far.