Refine CI Cache

[glhewett]

A couple of notes on the GitHub actions cache.

1. We can only create a cache, we do not update.
2. Caches for PR's do not apply to caches for main.
3. Caches should be invalidated when the vcpkg submodule changes or the vcpkg configuration changes.
4. Caches should have a version number that will force a rebuild of everything.