Add fix for Debian Bookworm/cloud-init/Netplan/DHCP issue

[jsf9k]

🗣 Description

This pull request adds a fix for the Debian Bookworm + cloud-init + netplan + DHCP issue described in cisagov/skeleton-packer#300.

[!IMPORTANT] This pull request cannot be merged until cisagov/skeleton-packer#242 is merged and trickles down to all the COOL AMI repos.

💭 Motivation and context

See cisagov/skeleton-packer#300.

🧪 Testing

I deployed these changes together with those from this branch of cisagov/debian-packer and verified that debiandesktop0 could ping terraformer0 without having to specify the .env6 domain.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.