Add support to OVERWRITE capz flatcar images to tekton pipelines

[primeroz]

Motivation

Follow up to #1659

Azure Gallery does not support overwriting an existing image so right now the build pipeline first check for an existing image matching the same FLATCAR_VERSION/KUBERNETES_VERSION combination and exit early if it exists

but we might need to rebuild an image under some circumstances like:

• Update replicated regions to create cluster in new regions
• Rebuild with an updated version of image-builder provisioning code
• others i can't think of now

as highlighted here we could

• delete the image version after checking for its existence when the OVERWRITE flag is set
  • but this is risky since we have no guarantee that the rebuild will work and we might end up with no image
• We could change the way that versions are created by
  • Create a temporary version ( FLATCAR_VERSION-build_timestamp )
  • have a task at the end of the build to replace the actual FLATCAR_VERSION with the built one as atomically as possible

TODO

• Implement a solution to the problem as described above ( or a better one )
• Document how overwrite works in capz pipelines

Outcome

For Clippy

• we can, when needed, rebuild the same images