EXP-853: Rebuild instances with PMIx as an external package

[mkandes]

EXP-853
expanse/0.17.3/cpu/b
expanse/0.17.3/gpu/b
tscc/0.17.3/cpu
tscc/0.17.3/gpu
YAML + SPEC

[mkandes]

Before we solve this issue in the existing production Spack instances, we will test the rebuild process by first completing the planned creation of a research, development, and test instances that is a core subset of the existing expanse/0.17.3/xpu/b productions instances. Once we've constructed this rdt instance, it will be deployed onto either Expanse or the Expanse Development cluster for testing the change before assessing the full impact and time required for a rip-and-replace rebuild of all packages with an MPI virtual dependency, which will likely be linked by default to PMIx as a dependency.