Make sure downstream projects get the right environment
Detector was internally setting PATH, LD_LIBRARY_PATH and ROOT_INCLUDE_PATH, but downstream projects were only seeing LD_LIBRARY_PATH, which was the only one needed until LHCb!3486 (merged). Now we also need ROOT_INCLUDE_PATH in downstream projects as we may need to include header files from Detector in a ROOT/cling environment.
This should fix Moore#417 (closed).