multi-arch manifest support of buildah images

Now that https://github.com/containers/buildah/issues/2724 is closed, we can finally publish a multi-arch image on quay.io, instead of having separate ones.

The test pipelines for the push are from the branch bentiss/ci-templates:wip/multi-arch-test (current one is at https://gitlab.freedesktop.org/bentiss/ci-templates/-/pipelines/278937).

The idea is that if we get a multi-arch manifest, we don't need to ship a @aarch64 variant, because the correct image will be picked up by the runner itself.

Note that qemu is still x86_64 only, but I am really tempted to use virtme or https://gitlab.freedesktop.org/mupuf/boot2container instead of bringing a full distribution for just running a few scripts. So long term plan is to drop the qemu variants in favor of something lighter.

Cc: @jadahl