[CI] Migrate the OSX cross build to use no script

Summary:
This diff make the osx CI build use the cross_build configuration.
For the build to succeed it is necessary to set the python library path
to use the library built from the depends. This used to be done in the
build-osx script, and has moved to the build_cmake.sh script; it is
now always set but will only be used for the osx build.

Depends on D7380.

Test Plan: Run the OSX CI build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7382