fix: clarify whoami api key source

[Bortlesboat]

Fixes PlatformNetwork/bounty-challenge#45898.

Aligns whoami's CORTEX_API_KEY auth-source wording with login status by including the environment variable label.

Verification:

• cargo test -p cortex-cli --test whoami_env_label -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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