GH action: fix routing of prefix setting (#1123)

Author: microbit-matt-hillsdon

.github/workflows/build.yml

@@ -46,6 +46,7 @@ jobs:
       - run: npm run ci
         env:
           REACT_APP_GA_MEASUREMENT_ID: ${{ secrets.GA_MEASUREMENT_ID }}
+          REACT_APP_GA_COOKIE_PREFIX: ${{ secrets.REACT_APP_GA_COOKIE_PREFIX }}
       - run: mkdir -p /tmp/app${PUBLIC_URL} && cp -r build/* /tmp/app${PUBLIC_URL} && npx serve --no-clipboard -l 3000 /tmp/app &
         if: env.STAGE == 'REVIEW' || env.STAGE == 'STAGING'
       - run: curl --insecure -4 --retry 7 --retry-connrefused http://localhost:3000 1>/dev/null