npm registry overrides should not be hardcoded

Follow on from the hack added in https://github.com/boltpkg/bolt/commit/69c1c55ba5aa007906e28156471e0f1988a00c57

We need to find a way to correctly load the users preferred npm registry.

We *should* be able to just use `npm config` to find it.

Start by reviewing the linked commit 69c1c55ba5aa007906e28156471e0f1988a00c57 to locate the hardcoded npm registry override. Then inspect how the CLI invokes npm and investigate `npm config` for the user's preferred registry; the work is done when that configured registry is loaded instead of the hardcoded value.

Written by the indexing model from the issue text.