trying to set dolfinx refs for ffcx testing for pull request

Author: sclaus2

.github/workflows/dolfinx-tests.yml

@@ -54,7 +54,7 @@ jobs:
         with:
           path: ./dolfinx
           repository: FEniCS/dolfinx
-          ref: main
+          ref: sclaus2/add-void-to-kernels
       - name: Get DOLFINx source (specified branch/tag)
         if: github.event_name == 'workflow_dispatch'
         uses: actions/checkout@v4