fix: Use custom domain for GITHUB_PAGES_URL instead of repository URL

The govuk-eleventy-plugin uses GITHUB_PAGES_URL to construct absolute URLs
for stylesheets. Using github.event.repository.html_url resulted in broken
CSS references pointing to https://github.com/co-cddo/ndx_try_aws_scenarios
instead of the actual GitHub Pages custom domain.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: chrisns

.github/workflows/build-deploy.yml

@@ -70,7 +70,7 @@ jobs:
       - name: Build Eleventy site
         run: npm run build
         env:
-          GITHUB_PAGES_URL: ${{ github.event.repository.html_url }}
+          GITHUB_PAGES_URL: https://aws.try.ndx.digital.cabinet-office.gov.uk
 
       - name: Upload build artifact
         uses: actions/upload-artifact@v4