Couldn't use pnpm



### Issue Summary

Couldn't use pnpm, but yarn works.

### To Reproduce

ModuleNotFoundError: Module not found: Error: Can't resolve '@gatsbyjs/reach-router/lib/utils' in '~/Documents/Example/React/gatsby-starter-ghost/.cache'

### Technical details:

* Ghost Version:
* Gatsby Version:
* Node Version:
* OS:
