Migrate image-download-check to new repo

[jonwinton]

Context: the main Kuberhealthy repo currently deploys the default Checks under the same tags as the primary controller. We'd like to separate Checks from the primary controller so that each Check and the controller can iterate separately.

Relates to: #1197

Move the image-download-check to its own repo once the the template repo is ready

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[github-actions]

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