GHA: OpenMP not found on macos

[dweindl]

e.g. https://github.com/AMICI-dev/AMICI/actions/runs/6637490835/job/18031913365#step:9:853

-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES) 
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES) 
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND) 

• ensure cmake can find the libomp installation
• update installation guide if relevant
• adapt test to always expect OpenMP to be available when running on GHA

[dweindl]

It's not so much an issue of mac, but rather clang. Reproducible on Ubuntu.

EDIT: okay on ubuntu it was just a missing libomp-dev; TODO: on mac try OpenMP_ROOT=$(brew --prefix)/opt/libomp