Make sure PUBLIC_URL is set in the sub-processes

Author: patch0

package.json

@@ -58,7 +58,7 @@
     "test:integration": "cd e2e; docker compose up --exit-code-from cypress",
     "start:wc": "NODE_ENV=development BABEL_ENV=development webpack serve -c ./webpack.component.config.js",
     "build:wc": "NODE_ENV=production BABEL_ENV=production webpack build -c ./webpack.component.config.js",
-    "heroku-postbuild": "PUBLIC_URL='' NODE_ENV=production BABEL_ENV=production yarn run build && yarn run build:wc"
+    "heroku-postbuild": "export PUBLIC_URL='' && yarn build && yarn build:wc"
   },
   "eslintConfig": {
     "extends": [