Fix building model extensions under conda on macos #3028
Description
Issues for the amici core extensions were fixed in #2839. However, importing the generated model extension still results in crashes on macos.
One issue seems to be that the package entrypoint shims for the cmake/swig/ninja PyPI packages installed with pip are put into some place other than $CONDA_PREFIX/bin, and thus, aren't used correctly. Maybe this can be resolved by pip install --prefix ... ... (untested).