Restore generation of LbEnv kit on SLC5
With the introduction of a PyPI proxy server (see LBCORE-1806), we can again prepare pip install LbEnv and the other tools.
Requires LbEnv!91 (merged) (in https://gitlab.cern.ch/lhcb-core/LbEnv/-/tags/1.1.5, waiting for https://gitlab.cern.ch/lhcb-core/LbEnv/pipelines/1133522 to complete)