Don't download any artifacts if JULIA_MPI_BINARY=system

MPI.jl seems to always download the MPICH and OpenMPI artifacts despite having set `JULIA_MPI_BINARY=system`. It is using the correct system MPI eventually but ideally it shouldn’t download the unused artifacts in the first place.

I guess / fear that this is a limitation of Pkg and the lack of optional dependencies. But I wanted to voice and archive this point somewhere and thought here would be the right place.