Keep the original behavior for "srun --mpi=pmix" by creating a symlink

on the most recent pmix_vX.so file.
One is free to directly use new "pmix_vX" plugin names as well.

NOTE: there is one small artifact introduced by this fix - there will be
no "pmix" name in the output of "srun --mpi=list" but the most recent plugin
will be duplicated:

$ ./srun --mpi=list
srun: MPI types are...
srun: mpi/none
srun: mpi/mvapich
srun: mpi/mpich1_shmem
srun: mpi/mpichgm
srun: mpi/pmix_v1      <----
srun: mpi/mpichmx
srun: mpi/lam
srun: mpi/pmi2
srun: mpi/openmpi
srun: mpi/mpich1_p4
srun: mpi/pmix_v1      <----