Set NODE_ENV=production when building for npm package

This fixes an issue where Babel's JSX transform was run in development mode when
building the npm package, bloating each JSX component with debug info.

Author: robertknight

.github/workflows/npm-publish.yml

@@ -18,6 +18,8 @@ jobs:
       run: yarn install --immutable
     - name: Build lib files
       run: yarn build
+      env:
+        NODE_ENV: production
     - name: Publish package
       run: npm publish --access public
       env: