(Try to) fix cloudbuild

[wongma7]

Is this a bug fix or adding new feature? fix cloudbuild.yaml

What is this PR about? / Why do we need it? the attempt to do multiarch cloudbuilds from #658 failed horribly with "Step #0: unknown flag: --tag". I think it has something to do with buildx not being available so I am shuffling that stray "HOME=/root" thing that I csopy/pasted from the doc to someplace that makes sense to me. Also sinc ethe github workflow has to enable multiarch using the qemu image then I guess this does too.

What testing is done?

docker run -e HOME=/root --entrypoint=bash -it  gcr.io/cloud-builders/docker
root@084b1a2e938b:/# DOCKER_CLI_EXPERIMENTAL=enabled docker buildx build --tag=gcr.io/k8s-staging-provider-aws/aws-ebs-csi-driver:latest .
[+] Building 0.0s (0/0)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wongma7

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[ayberk]

/lgtm

[coveralls]

Pull Request Test Coverage Report for Build 1388

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 81.713%

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Totals
Change from base Build 1386:	0.0%
Covered Lines:	1622
Relevant Lines:	1985

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💛 - Coveralls

[wongma7]

Progress, but not there yet.

Step #1: "docker buildx build" requires exactly 1 argument.

https://prow.k8s.io/view/gs/kubernetes-jenkins/logs/aws-ebs-csi-driver-push-images/1339014979134689280