Wrap build of DDG4 in a valid runtime environment

In some cases ROOT requires a valid runtime environment to be able to load libCore.so.

Similar to what happened in Gaudi (see gaudi/Gaudi!1419 (merged)), DD4hep invocation of listcomponents needs more than just LD_LIBRARY_PATH if the plugin library requires the initialization of the ROOT interpreter.

This MR wraps the invocation of cmake --build performed by ExternalProject_Add in an environment that is good enough for libCore.so to initialize without problems.