failed to read contents of npm cache

## Description

Error: ! failed to read contents of npm cache
at ChildProcess.onClose (/Users/.../.../.../.../.../node_modules/shrinkpack/src/analyse/read-npm-cache.js:40:25)
at emitTwo (events.js:126:13)
at ChildProcess.emit (events.js:214:7)
at maybeClose (internal/child_process.js:915:16)
at Socket.stream.socket.on (internal/child_process.js:336:11)
at emitOne (events.js:116:13)
at Socket.emit (events.js:211:7)
at Pipe._handle.close [as _onclose] (net.js:561:12)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! harmony@1.0.0 postshrinkwrap: `shrinkpack && node ./bin/zipTarballs.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the harmony@1.0.0 postshrinkwrap script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

## Suggested Solution

## Help Needed

Start with node_modules/shrinkpack/src/analyse/read-npm-cache.js at line 40 and the postshrinkwrap command shown in the report. Reproduction steps, environment details, and expected behavior are missing, so establish those first; done means the reported npm-cache workflow no longer fails and the case is covered by a regression test.

Written by the indexing model from the issue text.