cannot build new and old cmake projects mix in a workspace installed with lbn-install

the .xenv files generated with the new CMake projects rely on lb-run setting variables like <PROJECT>_ROOT_PATH, but this does not happen when we configure an old style project.

I stumbled on this issue when doing

lbn-install --platform x86_64_v2-centos7-gcc10-opt --project Panoramix lhcb-new-cmake/296
lb-set-platform x86_64_v2-centos7-gcc10-opt
cd lhcb-new-cmake/296/
lb-set-workspace .
cd Panoramix
lb-project-init 
make