Make sure envs of data packages are loaded

lhcb/LHCb!3127 (merged) was made under the false assumption that lb-run was not just locating data packages (!116 (merged)) but also including the respective .xenv files, leading to incorrect environments.

This (backward compatible) MR changes the behaviour of lb-run to make the lhcb/LHCb!3127 (merged) assumption true.