api_key_cmd is ignored if OPENAI_API_KEY is set

I use `op` as my `api_key_cmd`, but this only works if the OPENAI_API_KEY env var is not set.

I have exported my OPENAI_API_KEY to "op://Personal/OPENAI_API_KEY/credential" to be able to use the `op run` command to expand that variable and pass it to selected subprocesses, but this breaks the plugin because it seems that it defaults to get its api_key from env vars even if an `api_key_cmd` is set.

To me, the opposite behavior should be preferred.

No file or test is named in the issue. Trace how api_key_cmd and OPENAI_API_KEY are resolved, reproduce the behavior with both settings present, and add a regression test showing that the configured command takes precedence.

Written by the indexing model from the issue text.