Ensure PARSL_MPI_PREFIX is resolved for all MPI calculators

[Andrew-S-Rosen]

What would you like to report?

In #2197, we aimed to add support for the PARSL_MPI_PREFIX environment variable so resources are distributed appropriately for Parsl runs when using this new feature of Parsl. We should: 1) Make sure it works as-expected on HPC; 2) Expand the approach to other MPI calculators once we confirm it is fully operational. This is related to #2116.