GHA: `Tests MacOS Python` taking too long

[dweindl]

Tests MacOS Python GHA jobs currently take > 2h. Check what caused the increase, and consider splitting into multiple jobs. The time more than double sometime during the first half of August (https://github.com/AMICI-dev/AMICI/actions/workflows/test_python_cplusplus.yml?page=2&query=is%3Asuccess+branch%3Amain).

[dweindl]

last good: Jul 31, 11:46 AM GMT+2 (took 47m 46s) (The macos-latest label will migrate to macOS 15 beginning August 4, 2025. For more information see https://github.com/actions/runner-images/issues/12520
first bad: Aug 18, 7:49 AM GMT+2 (took 2h 5m 54s)

There weren't any amici code changes inbetween, so I guess, it's related to the change from macos-14 to macos-15 runners (changes). Maybe the clang version?

Others also experienced slower compilation: https://github.com/orgs/community/discussions/170682

[dweindl]

Definitely the runner image. Much faster with macos-14.

[dweindl]

For future reference: the default Clang/LLVM on macos-15 just changed from 16.0.0 to 17.0.0 (https://github.com/actions/runner-images/releases/tag/macos-15%2F20251103.0112). This may or may not resolve the issue.