storageAddCapabilities: fix bug in updating nursery pointers
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