Interleaved snapshot untar/rebuilding - Versioning

[apfitzge]

Problem

With #26590 we introduced snapshot rebuilding logic that relies on the version being 1.2 before we know that that's the case.

#27192 serializes the version file first, but we need to wait until that's rolled out before we can add version checking to code from #26590.

Proposed Solution

Deserialize version file before we start rebuilding (blocked by #27192 rollout) and pass the version up from there. This will allow us to rebuild from different versions while still unpacking/rebuilding in parallel