Downgrade oneprocess tests in the CI to ubuntu-22.04 because clang-format-18 is not yet supported

[valassi]

Debug github CI issues with clang-format

See https://github.com/madgraph5/madgraph4gpu/actions/runs/11304784032/job/31443586843

+++ Check code formatting in newly generated code ee_mumu.mad

ERROR! clang-format version >= 13 and <= 15 is not installed and /cvmfs/sft.cern.ch/lcg/releases/clang is not reachable
ERROR! mg-clang-format failed
ERROR! Auto-generated code does not respect formatting policies
 
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[testsuite_oneprocess.sh] codegen (ee_mumu.mad) finished with status=1 (NOT OK) at Sat Oct 12 10:07:47 UTC 2024
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Error: Process completed with exit code 1.

[valassi]

As expected, the version is too recent
https://github.com/madgraph5/madgraph4gpu/actions/runs/11305421978/job/31444906226?pr=1022

ERROR! clang-format version >= 13 and <= 15 is not installed and /cvmfs/sft.cern.ch/lcg/releases/clang is not reachable
ERROR! current clangVersion is '18'
ERROR! mg-clang-format failed
ERROR! Auto-generated code does not respect formatting policies

[valassi]

Hi @oliviermattelaer I self merge this because it is trivial and is necessary. All github CI tests fail otherwise.