fix: Amend CI so SITE_URL is undefined for live deployment

.gitlab-ci.yml

@@ -61,6 +61,7 @@ pages:
   extends: test
   stage: production
   script:
+    - unset SITE_URL # This is only used in the test job.
     - mkdocs build --strict --verbose --site-dir public
     # Expose services.yml for consumption by other apps/services.
     - mkdir -p public/api