Enable MPI pool on all CIs

[rasolca]

No description provided.

[rasolca]

cscs-ci run

[msimberg]

Any particular reason? Debugging? Consistency? I have nothing much against this, but just looking to understand the reasoning (as I think it can be good to test with it both on and off).

[rasolca]

Noticed during rebase of #1192 that pool was enabled only on CUDA pipelines with no clear reason.

IMO:

• Default behavior should be with pool.
• If we want to test without pool we can add or modify some pipeline (and should be tested on both mc and gpu)

[msimberg]

Already discussed separately with @rasolca, but making the request explicit: I'd like to keep at least one CI configuration with the pool disabled just as a sanity check, since the pool is off by default.