Should MPI.mpiexec() be ENV["MPITRAMPOLINE_MPIEXEC"] when MPIPreference.binary = MPITrampoline_jll?

[Sbozzolo]

I am using MPITrampoline_jll as my binary and I have set MPITRAMPOLINE_MPIEXEC. MPI.mpiexec() is picking up the artifact in my .julia, leading to failures. Shouldn't mpiexec always be MPITRAMPOLINE_MPIEXEC when using MPITrampoline?

[JBlaschke]

I principle, I am happy with this solution -- as it is the officially-documented workflow: https://github.com/eschnett/MPItrampoline

My question is if we should add some things to MPIPreferences -- e.g.:

1. a check to ensure that MPITRAMPOLINE_MPIEXEC is defined
2. a preference configure sensible defaults for MPITRAMPOLINE_MPIEXEC

The reason why I suggest these is so that one could configure the default behaviour of Julia in a single LocalPreferences.toml

[Sbozzolo]

This probably ties in with #801 ?

I don't know what a reasonable default for MPITRAMPOLINE_MPIEXEC might be.