Durable use cache: optimize env var existence checks (#98504)
We now separately track code that accesses non-inlined env vars at
runtime in two categories
- (existing) actual reads, the full value is accessed
- (new) only unset/falsy/truthy is read at runtime
For the second case, we only need to invalidate use-cache functions when
the given env var transitioned betwen unset/falsy/truthy. But
transitioning between two different truthy values doesn't caused
invalidation.
