Add flatcar build for Proxmox

[mcbenjemaa]

Change description

This PR adds:

• Flatcar build for Proxmox

Related issues

• Fixes #

Additional context

[k8s-ci-robot]

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[mcbenjemaa]

The build is working, but let me double check for any findings.

[mcbenjemaa]

WIP in CAPMOX
ionos-cloud/cluster-api-provider-proxmox#308

but I will provide a link to how this works in the provider once it is released.

[k8s-ci-robot]

@mcbenjemaa: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-azure-sigs	b79e31a	link	false	/test pull-azure-sigs

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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