x/build/cmd/relui: VERSION file CL submission race affecting RCs after early thaw

[dmitshur]

RC releases that happen after early thaw have a race: the relui workflow reads the head of the release branch at the start and does its work. During early thaw, the latest release branch is open to all approvers, and if any CL is submitted before the VERSION CL is, they'd need to be temporarily reverted and resent, or the relui workflow would need to be started anew.

So we need to temporarily freeze the release branch during such RC releases. This is the tracking issue for that.

CC @golang/release.

[heschi]

I think it might be fine, and certainly easier, to do the VERSION CL submission at the beginning of the workflow for RCs. For more sensitive releases we don't want to do externally-visible stuff until the release is ready to ship, but RCs are lower stakes.

[dmitshur]

That might be fine. It brings about some inflexibility and complexity overhead, too, which I imagine you're taking into account.