storageAddCapabilities: fix bug in updating nursery pointers

Summary:
We were unconditionally updating the nursery pointers to be
`nurseries[cap->no]`, but when using nursery chunks this might be
wrong. This manifested as a later assertion failure in allocate().

Test Plan: new test case

Reviewers: bgamari, niteria, erikd

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4649

(cherry picked from commit 4cb5595e)