Run Flatcar QEMU provisioning test in CI

[navaneeth-dev]

Goal

Add an end-to-end provisioning test that proves generated Butane output works on a real Flatcar VM.

Test path

1. Build bt.
2. Transpile a representative cloud-config fixture to a Butane config.
3. Translate it to Ignition with Butane strict validation.
4. Boot a pinned Flatcar image under QEMU.
5. Connect over SSH and verify the provisioned users, groups, home directory, shell, authorized keys, and locked-password behavior.
6. Fail on provisioning or boot errors and retain useful VM logs as workflow artifacts.

CI requirements

• Pin the Flatcar release used by the test.
• Verify downloaded release artifacts before booting them.
• Bound image download, boot, and SSH waits with explicit timeouts.
• Use KVM when available and document the fallback or skip behavior when it is not.
• Cache the Flatcar image where practical without weakening verification.
• Start as a scheduled or manually dispatched workflow if its download size and runtime make it unsuitable as a required pull-request check; promote it to a PR check only after measuring reliability and cost.

Completion criteria

The workflow provisions a Flatcar VM from bt output, checks the resulting OS state rather than only syntax, and provides enough logs to diagnose failures.