check.js: Please don't run checkNpm when disabled is true

[KernelLeak]

I was tearing my hair out why building a node.js app in Visual Studio 2019 was taking ages, and it turns out there was a node.js process running check.js that was hanging; killing it was making VS immediately finish it's build.

Long story short - I tried setting NO_UPDATE_NOTIFIER because I'm behind a corporate firewall and something was making direct requests to the repository, but it turns out it was package-json which is used indirectly in checkNpm (via latest-version); proxy support for package-json was shot down by it's maintainer, see the comedy that is sindresorhus/package-json#54 .

So even with NO_UPDATE_NOTIFIER set the check.js included with npm 6.10.2 included with the current node 12.9.1 out of the box hangs for a long time trying to access the registry when you're behind a proxy...