fix(ci): Write cached state images after update syncs, and use the latest image from any commit

[teor2345]

Motivation

Zebra's CI is slow, but we can speed it up by:

• writing images after update syncs
• using the latest image from any commit on any branch

We haven't seen any problems from sharing images between branches, so test speed is a higher priority.

Also close #4842

Solution

• Save cached state on full syncs and updates
• Add an -update suffix to CI images created by updating cached state
• Make disk image names unique by adding a time suffix
• Use the latest image from any branch, but prefer the current commit
• Document Zebra's continuous integration tests

Review

This is an urgent CI speed fix, but it might depend on a successful full sync creating new images on at least one branch.

Reviewer Checklist

• CI passes and is faster after the first run

[gustavovalverde]

LGTM