Update cloudbuild.yaml to run docker buildx
Update cloudbuild.yaml to run docker buildx

In README https://github.com/kubernetes/test-infra/blob/b74506333229b4cecfddccded988e80f5742e4b3/config/jobs/image-pushing/README.md
it's said to set HOME var for the docker buildx build

kubernetes/dns,kubernetes/dns,kubernetes/dns