Add explicit checkout of the coreCIF dictionary

Author: vaitkus

.github/workflows/main.yml

@@ -45,6 +45,12 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Checkout the coreCIF dictionary
+        uses: actions/checkout@v4
+        with:
+          repository: COMCIFS/cif_core
+          path: cif-dictionaries/cif_core
+
       - name: Run dictionary validity checks
         uses: COMCIFS/dictionary_check_action@main
         id: ddlm_check