Update MPI config

[charleskawczynski]

Based on the warning:

┌ Warning: The JULIA_MPI_BINARY environment variable is no longer used to configure the MPI binary.
│ Please use the MPIPreferences.jl package instead:
│ 
│     MPIPreferences.use_system_binary()  # use the system binary
│     MPIPreferences.use_jll_binary()     # use JLL binary
│ 
│ See https://juliaparallel.org/MPI.jl/stable/configuration/ for more details
│ 
│   ENV["JULIA_MPI_BINARY"] = "system"
│   MPIPreferences.binary = "MPICH_jll"
└ @ MPI /central/.../rrtmgp-ci/276/depot/cpu/packages/MPI/yvXmK/src/MPI.jl:103
*************************************************

This PR updates the MPI config

[sriharshakandala]

Is this dependency inherited from using ClimaComms? RRTMGP by itself does not use MPI!

[charleskawczynski]

Is this dependency inherited from using ClimaComms? RRTMGP by itself does not use MPI!

I'm not sure, I was just trying to fix the warning in the buildkite log. Maybe @simonbyrne better understands the warning?

[Sbozzolo]

Fixed in #444