Webpacker not respecting configuration

[coding-bunny]

Since #1395 got closed, continuing the discussion here because this problem needs to be fixed.

With the change introduced in d8dd446 , running rake assets:precompile breaks our CI since it always loads the config/webpack/production.js instead of the test.js file, causing the CI to error out since the code in that file does not have the required ENV variables available.

Explicitly setting NODE_ENV=test or RAILS_ENV=test has no bearing whatsoever on webpacker.
The only way we can currently get our assets compiled is by running RAILS_ENV=test NODE_ENV=test ./bin/webpack