ci: Fix extension version check on main (#935)

See CQCL/hugr#2374.


[error](https://github.com/CQCL/tket2/actions/runs/15875744214/job/44762805647#step:4:29)

Author: aborgna-q

.github/workflows/ci.yml

@@ -325,14 +325,20 @@ jobs:
           python-version: '3.10'
 
       - name: Check if extension versions are updated
-        # check against latest tag on the target branch
-        # base_ref should be empty if not a pull request, so goes from HEAD
         run: |
-            BASE_SHA=$(git rev-parse origin/${{ github.base_ref }})
-            LAST_TAG=$(git describe --abbrev=0 --match="tket2-*" $BASE_SHA)
+            # Check against latest tag on the target branch
+            # When not on a pull request, base_ref should be empty so we default to HEAD
+            if [ -z "$TARGET_REF" ]; then
+              BASE_SHA=""
+            else
+              BASE_SHA=$(git rev-parse origin/$TARGET_REF)
+            fi
+            LAST_TAG=$(git describe --abbrev=0 --match="tket2t c-*" $BASE_SHA)
             echo "Latest tag on target: $LAST_TAG"
-            
+
             python ./scripts/check_extension_versions.py $LAST_TAG
+        env:
+          TARGET_REF: ${{ github.base_ref }}
 
   # This is a meta job to mark successful completion of the required checks,
   # even if they are skipped due to no changes in the relevant files.