Fix LHAPDF link in the Madgraph gridpack
In the Madgraph gridpacks there is hardcoded symlink to LHAPDF data which can point to random location in nightlies if that was used to generate gridpack. At runtime we modify that based on actual runtime environment to point to CVMFS. It seems that with new CMAKE environment variable we use disappeared while another one is defined now. For some reason running locally still both are available thus Madgraph was happy locally while it failed in nightly. In this MR I move to use new variable.