Set mpirun to None when a single MPI task is being used (similar to handling of mpi-serial)

mpi-serial used to be required for building serially. It no longer is for modern MPI libraries. One of the benefits of mpi-serial is that it removes the complexity of having to use a mpirun command and options which is both different on different machines, adds complexity, makes it harder to port to a machine for a serial case, and often means you can't run a simple case on the command line.

There is special handling in CIME for mpi-serial that allows mpirun to be None. For this I think similar logic can be used to set it to None, in env_mach_specific.py in _find_best_mpirun_match.

The other way to handle this would be to add a setting for NTASKS==1 to config_machine.xml for the specific machines we want to do this for.