Update sysbox image in dind build test to fix docker API incompatibility.

The Problem:

* Test Container (Ubuntu Jammy): Uses curl -fsSL https://get.docker.com which
  installs the latest Docker version. As of March 2026, this is likely Docker
  27.x or 28.x, which uses API version 1.52+.

* Inner Container (ubuntu-focal-systemd-docker): Also uses curl -fsSL
  https://get.docker.com but was built earlier (likely with Ubuntu Focal repos
  from an earlier date). This probably has Docker 20.x or 21.x with API version
  1.41.

* When docker buildx create is executed from the outer container, it tries to
  connect via SSH to the Docker daemon inside the sysbox container. The buildx
  client (API 1.52) is too new for the Docker daemon (API 1.41) inside.

Fix by replacing ubuntu-focal image with newer ubuntu-jammy image.

Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>