github-action: use buildx

[v1v]

As explained in https://docs.docker.com/build/ci/github-actions/attestations/

otherwise

ERROR: Attestation is not supported for the docker driver.
Switch to a different driver, or turn on the containerd image store, and try again.
Learn more at https://docs.docker.com/go/attestations/
Error: buildx failed with: Learn more at https://docs.docker.com/go/attestations/

[v1v]

It works as expected:

• https://github.com/elastic/docker-attestation-demo/actions/runs/8877124523 tested this changes