`npm whoami` fails when `.npmrc` has custom registry

[fisker]

Description

Steps to reproduce

1. Add custom registry to .npmrc
2. Add publishConfig with NPM registry https://registry.npmjs.org/
3. Run np

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Expected behavior

np/source/prerequisite-tasks.js

Line 31 in 04dc206

externalRegistry: isExternalRegistry ? package_.publishConfig.registry : false,

should use registry from publishConfig even it's not external registry, otherwise NPM will read .npmrc

Environment

np - 10.2.0
Node.js - 10.2.0
npm - 10.7.0
Git - git version 2.42.0.windows.2
OS - Windows 10