When building extension packages against a production install of a

consuming application, it was likely that a build_pipeline_settings()

call in the app's settings.py would have path validation enabled,

looking for node_modules, and that the validation would fail.
In reality, we didn't care much about the presence of this path at this

stage, since the packaging process would be managing its own

node_modules. Rather than make the consuming app jump through hoops,

we now have a special environment variable that can be set to force

validation to be turned off. This mirrors the behavior of applications

using Djblets pre-2.0.
On top of this, we also explicitly define NODE_PATH to point to the

managed copy, ensuring it will be used instead of another, and avoiding

other lookup issues that could occur.

Tested building extensions against both a Review Board development server

and a production server.
Tested both initial builds (without a pre-populated node_modules directory)

and with a repeated build.