Possible lack of compatibility with Intel MPI implementation

[bhazelton]

This hasn't been thoroughly investigated, but we saw some test failures recently on #450 and #451 that seemed to be caused by mamba picking up the impi package rather than either openmpi or mpich (both of which ran without errors). To fix those errors, we explicitly set the MPI implementation in our test yamls in #450 and we are now testing against both Open MPI and MPICH in our CI but not against Intel MPI.