npm should always be used with NO_UPDATE_NOTIFIER setted

Since cordova-lib is using npm commands, when I am behind a corporate proxy npm is trying to check if npm has new versions available and get stuck for some time each time npm command is spawned.

As a workaround, I set NO_UPDATE_NOTIFIER environment variable manually to disable the check done by the "update-notifier" module which is used by npm.

I think that cordova-lib should set this variable itself when it spawn npm commands since this check is probably unnecessary.