WIP: fix (probable) race condition in promotion

[justinsb]

I believe there is a race condition here, and we are closing the channel
during promotion before we finish filling it, in ExecRequests.

This manifests as images not being promoted, seemingly at random,
without errors.

Refactor to make the logic (hopefully) clearer.

Simplify code to address potential race condition that was (likely) causing incomplete image promotion.

[k8s-ci-robot]

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[justinsb]

This is not the most idiomatic, but preserves indentation for easier review.

[k8s-ci-robot]

@justinsb: 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-cip-verify	9cd2dbf	link	true	/test pull-cip-verify

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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[k8s-ci-robot]

PR needs rebase.

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