fix(ci): set `GH_PAGES` to `true` for storybook build

.github/workflows/deploy-storybook.yaml

@@ -40,6 +40,8 @@ jobs:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}
     runs-on: ubuntu-latest
+    env:
+      GH_PAGES: true
     needs: cache-dependencies
     steps:
       - uses: actions/checkout@v2