Cleanup build-driver config setting

[carolynvs]

What does this change

I found a few spots I missed when removing docker from the build-driver options.

What issue does it fix

Follow-up to #1999

Notes for the reviewer

We have plans for adding another build driver in the future: podman. That's why we aren't just removing the build-driver option entirely.

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR