Allow environment variables to set CLI defaults

[bittner]

Adds a few environment variable getters that allow to override the default value of some CLI options.

Fixes #754

[bittner]

@woodruffw I've not yet covered documenting the environment variables. Not sure what the best way would be to do it. A multi-line explanation in argparse.ArgumentParser(description) could be one possibility doesn't work.

[woodruffw]

Thanks @bittner, this looks like a great initial set!

I've not yet covered documenting the environment variables. Not sure what the best way would be to do it.

Hmm, IMO it'd be fine to document it in the README alone, i.e. under the "Usage" section in a new sub-section above the "Exit codes" one: https://github.com/pypa/pip-audit?tab=readme-ov-file#exit-codes

(A short explainer that some flags honor the environment + a bullet list of each of them would be perfect.)