bleed npm registry config through to builder

right now, the builder doesn't get the registry passed to it, and therefore uses the default.

I don't think anyone is configuring this right now so it's not a huge deal.

Start by locating where the npm registry configuration is read and where the builder is created or invoked. Trace whether the configured registry reaches the builder instead of falling back to npm's default, and verify that the builder uses the passed registry.

Written by the indexing model from the issue text.