Durable use cache: fix runtime env var mutation (#98558)
### Problem
1. Next computes the warmup cache key while `SOME_ENV_VAR` is unset.
2. Rendering ends up setting `SOME_ENV_VAR`.
3. Final prerender expects the cache entry to exist, but the env var
changed, so the key changed and it's a miss.
4. Unexpected miss and bailout.
This lead to
```
Error: Route "foo": Unexpected cache miss after cache warming phase during prerendering. This is
likely caused by non-deterministic arguments that differ between the cache warming phase and the
final prerender phase (e.g. unstable array order). Ensure that arguments passed to cached functions
are deterministic.
Error: Route "foo": Next.js encountered uncached or runtime data during
prerendering.
```
### Solution
~~Instead, snapshot the env vars once at module evaluation time of the
`use cache` function. This ensures that the cache key doesn't change
over time. This is also how it worked thus far (with deployment id as
the cache key): changing the env var over the lifetime of the process
didn't lead to a reexecution of the `use cache` function~~
RDC already stores cache entries generated in the previous phase (be it
during `next build` multi-phase rendering, or from prerender->resuming
at runtime). Root params are already excluded from the cache keys when
storing in RDC. Also exclude the env var hash bit, to conform to this
system of preventing tearing (at the cost of potential staleness).
