Fix/macos all runtimes

[RobertSamoilescu]

This PR fixes the all-runtimes tests for macos and reduces flakiness for the other tests.

Related to https://github.com/SeldonIO/MLServer/pull/1184 Seems that there is work in progress wrt to this issue on both lightgbm and xgboost. Hopefully, in the near future we can simplify the installation of OpenMP and use the latest version:

• https://github.com/microsoft/LightGBM/issues/4229#issuecomment-2169130455 (released in lightgbm==4.4.0)
• https://github.com/dmlc/xgboost/commit/45150a844e6c4aa90746f7af724ee2c729f90def and https://github.com/dmlc/xgboost/commit/8689f0b5621f166dbb6be6983eb897688cc0485b (not yet released)

Other relevant links:

• https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories