use cffconvert directly

Author: wpbonelli

.github/workflows/ci.yml

@@ -66,7 +66,7 @@ jobs:
         run: pixi run check-python-format
 
       - name: Check CITATION.cff
-        uses: dieghernan/cff-validator@v3
+        run: cffconvert --validate -i CITATION.cff
 
   build:
     name: Build

environment.yml

@@ -7,6 +7,7 @@ channels:
 dependencies:
   - python>=3.9
   - appdirs
+  - cffconvert
   - codespell
   - filelock
   - flaky