Fix nlohmann_json dependency on Ubuntu, and the CI pipelines
This MR fixes the dependency on nlohmann_json on Ubuntu by introducing a standalone build of the dependency package with the CMake option: -DGEOMODEL_USE_BUILTIN_JSON=TRUE
The official nlohmann_json package for Ubuntu, in fact, is too old (v. 2.x) for the methods we use in GXClasPointsSystems, which have been introduced in v. 3.3. Thus, on Ubuntu, users can now build nlohmann_json as part of the build. This is now the option used for the Ubuntu CI pipeline, as well.
This MR fixes the macOS CI pipeline, too.