Update windows.yml

.github/workflows/windows.yml

@@ -142,7 +142,7 @@ jobs:
         working-directory: dolfinx
         run: |
           pip install pytest-xdist
-          python -m pytest -n auto -m "not petsc4py and not adios2" python/test/unit
+          mpiexec -n 1 python -m pytest -m "not petsc4py and not adios2" python/test/unit
       - name: Run units tests (Python, MPI, np=3)
         working-directory: dolfinx
         run: |