WorkDir: only setup cppcheck if it can be found within ALRB

On the lxplus Alma9/RHEL9 nodes, we have cppcheck (2.9) installed by default. Change WorkDir such that it only enables cppcheck if it has been found via ALRB, i.e. the user explicitly enabled it via lsetup cppcheck. This restores the same behavior as on CC7 where we don't have cppcheck locally installed. This also gives us the possibility to enforce a specific cppcheck version if needed.

Relates to ATLINFR-2654. cc @akraszna