[guix] Added package openmpi-pmix.

Then you can build portable (tested eve end juwels) container with:

guix pack -L $PWD/.guix/modules --fallback --without-tests=hdf5-parallel-openmpi --with-input=openmpi=openmpi-pmix -RR --format=squashfs bash ogs-petsc

On Juwels you need to start with `srun --mpi=mspmix ...`