`build_images` firestartr workflow is overwriting multi-arch images manifests

[alambike]

Description

After #209, we have the ability to filter the platforms which an image should be built in an execution, but when building the same image, with different platforms, from different jobs (to be able to use different runners according to the platform/arch), the last image built is overwriting the multi-arch manifest, causing that all other platforms are lost.

We need to query the registry and load the image's variants already available, before publish() the new image, to don't lose them.