set baseurl to match exposed artifact preview

This makes links like stylesheets etc match where they get served up for
exposed artifacts, rendering things a bit more correctly.

.gitlab-ci.yml

@@ -3,7 +3,8 @@ image: registry.gitlab.com/pages/hugo:latest
 
 test:
   script:
-  - hugo --buildDrafts --buildFuture
+  - BASE_URL="https://$CI_PROJECT_NAMESPACE.$CI_PAGES_DOMAIN/-/$CI_PROJECT_NAME/-/jobs/$CI_JOB_ID/artifacts/public/"
+  - hugo --buildDrafts --buildFuture --baseURL=$BASE_URL
   artifacts:
     expose_as: 'Preview changes'
     paths: