Support credentials via OPENAI_API_KEY environment variable

[dbosk]

Basically all packages that use the OpenAI APIs support supplying the API key through the OPENAI_API_KEY environment variable. It would be nice if smartcat joined that convention. (Unfortunately, my Rust is too rusty, otherwise I would have made a PR.)

A nice consequence of adding this is that smartcat can then be used more easily inside Docker containers. It's must easier to pass an environment variable than to load a config file into the right location on the fly.