fix: Hide LLM api key in generated code

[hughlv]

Theoratically it's fine to include the custom api-key in the generated code, however since it can be download can spread to other people. There is risk of api-key leakage.

config_list = [
    {"model": "gpt-4-1106-preview", "api_key": os.environ["OPENAI_API_KEY"]},
    {"model": "gpt-3.5-turbo", "api_key": os.environ["OPENAI_API_KEY"]},
]

This is the general solution that hides value of OPENAI_API_KEY, however it will make it impractical to add new api keys on UI, because we're not supposed to modify .env variables programatically.