CI: don't embed SSH keys in the rootfs

[pb8o]

Changes

• Update rootfs to Ubuntu 24.04
• Avoid the need to create an ext4 image
• Avoid embedding SSH keys in the rootfs. We still bake in a key before running tests.

Reason

Not hardcoding the key in the rootfs decreases the changes the key is reused in some other context.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check CONTRIBUTING.md.

PR Checklist

• [x] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [x] API changes follow the Runbook for Firecracker API changes.
• [x] User-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.
• [x] New TODOs link to an issue.
• [x] Commits meet contribution quality standards.

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• [x] This functionality cannot be added in rust-vmm.