ci(cffconvert): introduce cffconvert job to ensure valid CFF file

.gitlab-ci.yml

@@ -119,6 +119,11 @@ black:
     - poetry install --only dev
     - black --check .
 
+cffconvert:
+  script:
+    - pip install --upgrade cffconvert
+    - cffconvert --validate
+
 release:
   rules:
     - if: $CI_COMMIT_TAG == null