nsxqt does not start with invalid RPATH

I found the following scenario that triggers nsxqt not to start on the new CI machine ci-sci-ubuntu-1:

• gsl 2.1 is installed by neighboring projects (e.g. Mantid)
• I installed locally gsl-2.3 which is needed by libnsx

After investigating, I found that CMake is not using the RPATH used in the build tree when being installed. Look here for more explanations