fix: set webpack environment variables.

[dianakhuang]

This will hopefully fix some issues we're seeing in building static assets.

---

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.

[robrap]

Should I be comparing these values to anything, or just give you a thumb so you can see if it works? :)

[dianakhuang]

@robrap we're seeing webpack errors, and the exact message in the paver version is:

WARNING: 'paver webpack' is DEPRECATED! It will be removed before Sumac.
The command you ran is now just a temporary wrapper around a new,
supported command, which you should use instead:

	WEBPACK_CONFIG_PATH=webpack.prod.config.js NODE_ENV=production STATIC_ROOT_LMS=/edx/var/edxapp/staticfiles STATIC_ROOT_CMS=/edx/var/edxapp/staticfiles/studio JS_ENV_EXTRA_CONFIG="{}" npm run webpack

Details: https://github.com/openedx/edx-platform/issues/31895