Set cluster version on the extended channel

[julianKatz]

What type of PR is this?

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/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation

/kind failing-test

/kind feature
/kind flake

What this PR does / why we need it:

Tests on the extended channel are currently failing because "No valid versions with the prefix "1.28" found". Those are no longer available outside of the extended channel. Thus, I reasoned that the extended channel variable was not being set.

When I looked in the clusterUp golang logic, I found logic that wouldn't set the release channel if a version was specified. There is now a special case for the extended channel.

Does this PR introduce a user-facing change?:

None

[k8s-ci-robot]

Hi @julianKatz. Thanks for your PR.

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[mattcary]

/ok-to-test
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julianKatz, mattcary

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