xz compression is used for the ami and tar output types, it should be in
the buildroot. This fixes Weldr integration test (/cmd/osbuild-tests) on
ARM architecture (aarch64).
The fact that it worked on x86_64 was just a happy coincidence because
we require grub2-pc which in turn requires dracut which requires xz. We
should not rely on these implicit dependencies because we need xz
unconditionaly, therefore adding it to build packages for all platforms.
e1e5c1bb0b