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

<!--- Provide a short summary of the issue in the title above --> ## Description <!-- Write a clear and concise description of what the issue is --> ## Steps to reproduce <!--- Let us how how we can reproduce the issue on our end --> 1. Add custom registry to `.npmrc` 2. Add `publishConfig` with NPM registry `https://registry.npmjs.org/` 3. Run `np` ``` You must be logged in. Use `npm login` and try again. ``` ## Expected behavior <!--- Tell us what you expected to happen --> https://github.com/sindresorhus/np/blob/04dc20651316ba4697ac998bfe864cf53c776b83/source/prerequisite-tasks.js#L31 should use registry from `publishConfig` even it's not external registry, otherwise NPM will read `.npmrc` ## Environment <!-- Mention which versions of np, Node.js, npm and Git you're using, as well as your OS and version --> np - 10.2.0 Node.js - 10.2.0 npm - 10.7.0 Git - git version 2.42.0.windows.2 OS - Windows 10