Don't run mirgecom downstream CI in parallel (gh-211)

.github/workflows/ci.yml

@@ -116,6 +116,9 @@ jobs:
                     # can't turn off MPI in mirgecom
                     export CONDA_ENVIRONMENT=conda-env.yml
                     echo "- mpi4py" >> "$CONDA_ENVIRONMENT"
+                    
+                    # https://github.com/inducer/grudge/issues/211
+                    export CISUPPORT_PARALLEL_PYTEST=no
                 else
                     sed -i "/mpi4py/ d" requirements.txt
                 fi