Migrate MPI CI

[rhodrin]

Move back github hosted runners for the time being.

Note that workflow is being kept in the current format since we should be moving this back to our own cluster in the new year.

[FabioLuporini]

uncontroversial

[codecov]

Codecov Report

Merging #1531 (c822065) into master (bab25a1) will decrease coverage by 26.16%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #1531       +/-   ##
===========================================
- Coverage   87.15%   60.99%   -26.17%     
===========================================
  Files         203      183       -20     
  Lines       28898    27170     -1728     
  Branches     3910     3698      -212     
===========================================
- Hits        25186    16572     -8614     
- Misses       3274    10042     +6768     
- Partials      438      556      +118     

Impacted Files	Coverage Δ
examples/seismic/self_adjoint/operators.py	9.23% <0.00%> (-90.77%)	⬇️
tests/test_dimension.py	9.37% <0.00%> (-90.63%)	⬇️
tests/test_caching.py	13.50% <0.00%> (-86.25%)	⬇️
tests/test_dse.py	14.41% <0.00%> (-85.21%)	⬇️
tests/test_operator.py	13.00% <0.00%> (-84.81%)	⬇️
tests/test_checkpointing.py	15.21% <0.00%> (-84.79%)	⬇️
tests/test_differentiable.py	15.38% <0.00%> (-84.62%)	⬇️
tests/test_symbolic_coefficients.py	16.41% <0.00%> (-83.59%)	⬇️
tests/test_staggered_utils.py	16.66% <0.00%> (-83.34%)	⬇️
tests/test_subdomains.py	17.44% <0.00%> (-82.56%)	⬇️
... and 143 more

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[FabioLuporini]

Thanks, merging