Cannot `using Trixi` on a cluster (RAMSES)

I'm on a node of the UoC's cluster RAMSES, and I get this:
```
julia> using Trixi
slurmstepd: error: mpi/pmi2: invalid pmi1 command received: 'init'
```
It just freezes there.
This is coming from `init_mpi()` in `__init__()`.

Trixi.jl is not usable on this system, as it doesn't get past the initialization.

Start at init_mpi() in Trixi.jl's __init__() and reproduce the failure on the RAMSES cluster under Slurm. Investigate the reported PMI2 command error; done means using Trixi completes initialization on that system without freezing.

Written by the indexing model from the issue text.