attestation: show unit of time for retry message

[carlocab]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
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• Have you successfully run brew tests with your changes locally?

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Otherwise this just shows a message like

Failed to verify attestation. Retrying in 27...

which is a little vague. Let's make it clear that that refers to
seconds.