fix(nest): Align MPI variant for hdf5 dependency
@terhorst This fixes the problem where a hdf5+mpi and a nest@3.6~mpi lead to build errors by requiring a matching +mpi/~mpi variant for hdf5/nest.
@terhorst This fixes the problem where a hdf5+mpi and a nest@3.6~mpi lead to build errors by requiring a matching +mpi/~mpi variant for hdf5/nest.