Simplify installation of az CLI on Windows

[mboersma]

What this PR does / why we need it:

Simplifies installation of the az CLI on Windows, which after initial testing seems to be more reliable than the download -> install -> delete approach image-builder currently uses.

Which issue(s) this PR fixes:
Fixes #1355

Additional context
Add any other context for the reviewers

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[mboersma]

/test pull-azure-sigs

[k8s-ci-robot]

@mboersma: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-azure-sigs	fee8a51	link	false	/test pull-azure-sigs

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[mboersma]

/close

This didn't work, apparently.

[k8s-ci-robot]

@mboersma: Closed this PR.

In response to this:

/close

This didn't work, apparently.

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