Fix schema error in kustomize/cloudbuild.yaml

[xak2000]

I don't know how this happened but PR #476 introduced a schema error to kustomize/cloudbuild.yaml.

For some reason it sets steps[0].env property as string:

env: 'KUSTOMIZE_VERSION=${_KUSTOMIZE_VERSION}'

But env is expected to be an array, so CloudBuild can't read the cloudbuild.yaml file and shows this error when build is triggered:

Your build failed to run: failed unmarshalling build config kustomize/cloudbuild.yaml: json: cannot unmarshal string into Go value of type []json.RawMessage

Maybe CloudBuild had accepted a string as env before, I don't know, but now it requires an array. So, this PR fixes this.

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