Fix build with CMake "Ninja Multi-Config" generator

As of CMake 3.17 the Ninja Multi-Config generator can be used to produce a build directory that can compile for multiple CMAKE_BUILD_TYPEs at the same time.

Unfortunately we cannot use it out of the box in Gaudi because of the special environment scripts we need at build time (run and gaudienv.sh), which should be generated in multiple copies, one per build type.

To properly support multi-config generators, we need to review the way we deal with the runtime environment needed at build time.