ci: Enable assertions in CMake build jobs on macOS

We did this previously on Linux and Windows:

* commit 45feab3c00 (gitlab-ci: enable assertions on fedora34-ninja jobs,
                     2022-01-31, v3.23.0-rc1~37^2)
* commit bec34592a6 (ci: Enable assertions in CMake build job on Windows,
                     2024-12-19)

The macOS build jobs were already building the `Release` configuration
by default.  Make it explicit to be sure we use these flags.