Cross compile doesn't support podman

[awels]

Describe the bug
My development machine used only podman and the cross build options use buildx which only work for docker. Podman has a different mechanism for cross builds.

Steps to reproduce
Try to build the project on a pure podman/buildah setup.

Expected behavior
Ability to build the project with podman

Actual results
failure to build because buildx is used and it doesn't exist with podman.

Additional context
Some discussion on how to fix it.
containers/buildah#1590

[tesshuflower]

As far as I know, the -buildx make target was added by operator-sdk. We're not currently using it in our builds or automation. Wonder if it's worth seeing if operator-sdk intends on supporting podman...