ensure that npm environmental config gets used

Fixes npm/npm#6483.

/cc @iarna for review / knowledge-sharing @wilmoore because he identified this bug.
> Also, @rvagg and @timoxley lobbied pretty hard for that change, so you'd have to convince them before I'd tempt their wrath by changing the default again.

Ah...and I'm sure they didn't come to the conclusion that they did lightly. Fair enough :+1: