ci: gate storybook build workflow on fork

Add the same repo guard applied to sibling upstream workflows in
bd6241ad9d. On the private fork this job failed at checkout because
permissions: {} leaves GITHUB_TOKEN without contents:read, so
actions/checkout 404s on the private repo.

.github/workflows/shared-component-storybook-build.yml

@@ -8,6 +8,7 @@ permissions: {}
 
 jobs:
     doc:
+        if: ${{ github.repository == 'element-hq/element-web' }}
         name: Build storybook
         runs-on: ubuntu-latest
         steps: