Adapt CMake build system's "external Acts" configuration
The CMake configuration for external Acts installations seems to differ from the one for the internal submodule:
- It references the
MaterialMappingPlugin
component, which does not exist anymore. - It does not reference the JSON plugin, which the internal build seems to use now.
The two configurations should probably be brought back in sync. It might also be a good idea to have the CI test the external Acts installation in order to avoid such divergence in the future.