Revert "Uninstall pytest-mpi"

This reverts commit 98bb9d7.

.github/workflows/build.yml

@@ -89,8 +89,7 @@ jobs:
           . ../firedrake_venv/bin/activate
           python "$(which firedrake-clean)"
           python -m pip install \
-          pytest-xdist pytest-timeout ipympl
-          python -m pip uninstall -y pytest-mpi
+            pytest-xdist pytest-timeout ipympl
           python -m pip list
       - name: Test Firedrake Serial
         run: |