Set prerelease=true for new releases before images are ready

[wongma7]

Is this a bug fix or adding new feature?

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

This is a hack to "hide" the release until images are ready. Right now, we cut a github release the moment a a tag is pushed. However, there is a period after that during which images need to be scanned and possibly rebuilt. Therefore we should treat the release as "pre-release" until we actually have a working image that passes tests/scans etc., at the end of the release process.

What testing is done?

[k8s-ci-robot]

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[wongma7]

/assign @gtxu

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[nckturner]

/lgtm