Run unstable_cache inside a CacheStore context (#70740)
The idea is that CacheStore context will merge with Action, Prerender
and Request Context but this just makes a disjoint union of the two
Caches unstable_cache and "use cache".
This replaces the `isUnstableCacheCallback: true` and `fetchCache:
'force-no-store'` override. Instead, we need to check manually whether
we're in a cache context.
This lets us avoid cloning the WorkStore context object. Now there's
only one copy which makes it safer to mutate it.
