unstable/options: fix for `chaperone-struct` change
unstable/options: fix for `chaperone-struct` change

Contrary to the comment for 0b71b8481d, the `unstable/options`
implementation used the first argument to a mutator wrapper.
Adjust to close over the wrapped value.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket