`lib.recursiveUpdate` indiscriminately recurses into all attribute sets,
also into derivations. This means that it is possible that evaluating a
derivation in the final haskell package set can cause something in
`prev.haskell` to be forced by `recursiveUpdate`, potentially causing an
evaluation error that should not happen.
It can be fixed using a well-crafted predicate for
`lib.recursiveUpdateUntil`, but most robust is just explicitly writing
out the desired merging manually.