fix(tekton): revert to compression after build-image-index

The Tekton typed array result expansion doesn't work correctly when
passing to build-image-index IMAGES parameter. The array result from
convert-compression task was only passing the last element.

Revert to the simpler approach where compression happens AFTER
build-image-index. When buildah manifest push is called with
--compression-format --force-compression --all, it re-pushes all
architecture images with the new compression.

Author: jiridanek