Fix `ReadRef<T>::cell` when `T` != `T::Read::Repr` (#8845)
## Description
Fixes the test case introduced in #8843.
The theory is that Vcs are supposed to be stored as `<<T as
VcValueType>::Read as VcRead<T>>::Repr`. However, it looks like due to
an oversight, `ReadRef::cell` is trying to store the value as `T`.
This introduces a way to perform
## Testing Instructions
```
cargo nextest r -p turbo-tasks -p turbo-tasks-memory
```
