fix: add env config handling in patch of webpack prod config

[jsnwesson]

APER-2786

This is a follow-up to #275 to have Webpack Prod config use env.config.js for assigning NEW_RELIC_APP_ID and BASE_URL during the creation of two New Relic plugins.

This work is a temporary patch until we can add these changes to frontend-build and Learner Record migrates from using @edx/frontend-build to @openedx/frontend-build as a dependency.

Closes #281

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fc17b47) 70.22% compared to head (f6604b8) 70.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   70.22%   70.22%           
=======================================
  Files          27       27           
  Lines         403      403           
  Branches       85       85           
=======================================
  Hits          283      283           
  Misses        119      119           
  Partials        1        1           

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