Djblets 5 introduced a new architecture for building extension packages,

with an eye toward building new, non-setuptools-based package backends.

As part of this, most of the static media and environment setup flows

changed.
This introduced a regression when building against a production set of

packages, rather than a development environment. We were setting the

environment variable DJBLETS_SKIP_PIPELINE_VALIDATION=1 too late,

after trying to set up Django.
The problem is that we needed this variable in order to avoid validating

node_modules-based paths during Django environment setup. The

packaging process then failed early, never getting to the point where we

actually set up the Node environment for static media building for that

package.
This setting has been moved back to the Setuptools backend before

setting up the Django environment, back where it was before.

Unit tests passed.
Installed Review Board via production packages and then built an extension

against it from a clean source tree. Previously, this was failing. After

this change, node_modules populated, static media was compiled, and the

package built successfully.