replace --prefer-profile with --prefer-env-api-key

[viadezo1er]

At the moment, unless --prefer-profile is passed, a BRAINTRUST_API_KEY in the env takes precedence over the logged-in profile.
This is probably a footgun.

Replace --prefer-profile with --prefer-env-api-key: if this flag is not passed, prefer the logged-in profile if it exists. If it is passed, prefer the BRAINTRUST_API_KEY if it is passed.
In both cases auth uses the non preferred method of authentication if the preferred one is not available.

Thanks to Alex Steere for surfacing this.