Old PRs can add a new "latest" cached state w/ the old state version

[dconnolly]

Motivation

We don't check the state version when updating the cached state, so old PRs can add a new "latest" cached state with the old state version. This won't break CI, but it adds 2.5 hours to the mergify queue.

We can check cached state version vs the version in the current branch being tested. Doing this requires making the cached state version easier to find in CI. One idea is adding the state version as disk image metadata or in the name of the disk, and then doing some Rust test / grep check to compare the latest cached state disk version with the in-branch version, and only using the cached state if they are the same version.

If the cached state version is higher than the branch version, we can not run the test, failing the test until the branch is rebased onto #main.

If the cached state version is lower than the branch version, we can run the test from an empty disk? Requires a full sync to the stop point.

Either way, the goal is that 'old' state versions (older than the version of state on #main) should not be created and pushed to gcloud, where they can be used by other tests.