Rename NODE_ENV to WEBPACK_BUNDLE (#18087)

NODE_ENV interferes with other tools (e.g. `yarn` won't pull
devDependencies). Since what we need is only a flag indicating Webpack
to bundle for production vs developement, invent our own env.

This also solves the mystery of unable to call `yarn build` in the build
script `bin/build-mb/src/build.clj`.