Fix stray argument in cloudbuild.yaml

[wongma7]

Is this a bug fix or adding new feature? fix

What is this PR about? / Why do we need it? this explains the

ERROR: (gcloud.builds.submit) build 91a9a509-a135-44aa-a439-af84587bbd96 completed with status "FAILURE"
Step #1: "docker buildx build" requires exactly 1 argument. 

error.

I still intend to move to a model where the cloudbuild calls a script similar to https://github.com/kubernetes-csi/csi-release-tools/blob/master/prow.sh#L1243 https://github.com/kubernetes-csi/csi-release-tools/blob/master/build.make#L132 (ty to person who pointed me to these : ) ) , it will be easier to maintain than wrestling with cloudbuild syntax, and anyway the github workflow could call the same script so we don't have to maintain two copies of the same code. This is just a nobrainer quickfix.

What testing is done?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ayberk, wongma7

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[ayberk]

/lgtm

[coveralls]

Pull Request Test Coverage Report for Build 1390

• 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 1389:	0.0%
Covered Lines:	1622
Relevant Lines:	1985

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💛 - Coveralls