CMake modernization broke `<Project>-build.xenv` hack

The CMake modernization got rid of xenv. A local environment for a given project is now done by a generated shell script <project>env.sh, which is using the InstallArea of the dependencies.

As a result, the following cross-project things are broken:

1. A change in Python in LHCb requires running make so that the effect is visible in Moore/run.
2. Cannot set a breakpoint in LHCb while starting gaudirun.py in Moore.
3. Navigating through sources across projects sometimes sends you to the InstallArea.
4. ?

There is no known workaround. We can try to transition to an all-in-one master build. To be seen how this plays with projects that are not yet migrated to the new CMake.