bug#73649: OpenMPI + mpi4py broken: rank=0

[Antero Mejr]

Running this command:

guix shell python python-mpi4py openmpi -- mpirun -n 12 python3 \
-c "from mpi4py import MPI; comm = MPI.COMM_WORLD;
print(comm.Get_rank())"

should print the numbers 0-11. Instead it prints 0 11 times. The same
behavior occurs in a container or pure environment.

This breaks any code using mpi4py, because ranks >0 must be available
for multi-threaded code to work.