v2024dev1: Mark OVA generation action as a postprocess task

The action run to generate the ova image isn't currently marked as a postprocess task, so it's being run when the image is still mounted. This has oddly seems to be working in most places, that is until it stopped in a teams use of this recipe resulting in missing boot config files.

Mark this action as a postprocess step to avoid racy behaviour.

Same as !588 (merged) but for v2024dev1.