Detecting context in interactive slurm job assumes MPI even when it shouldn't

[Sbozzolo]

ClimaComms.context() checks whether the environmental variable PMI_RANK is defined. If it exists, ClimaComms will assume that we are in a MPI job. This is breaks when running interactively in a slurm system, as the variable is created even if the user has requested one task and has no intention on using MPI.

To reproduce, just ask for a thread with srun and run ClimaComms.context().

I don't know what's the cleanest way to fix this.