[enh] ci: `pages` now create a coverage artifact

.gitlab-ci.yml

@@ -82,6 +82,10 @@ test:
     # Code coverage display in GitLab
     - sed -n 23p coverage/index.html | grep -Po "\d+.\d+" | sed -e "s/\(.*\)/<coverage>\1%<\/coverage>/"
   coverage: '/<coverage>(\d+.\d+\%)<\/coverage>/'
+  artifacts:
+    paths: coverage
+    expire_in: 4h
+
 .build_releases: &build_releases
   stage: package
   allow_failure: false