Configuration failure because of missing xenv in new nightlies

When running in new nightlies, LHCbIntegrationTests fails in configure step because xenv can't be found, see e.g. https://lhcb-nightlies.web.cern.ch/testing/lhcb-head/2272/LHCbIntegrationTests/x86_64_v2-centos7-gcc11-opt/build.

Instead of adding xenv to the nightlies environment, one could pull xenv from git using this trick: https://gitlab.cern.ch/lhcb-core/LbDevTools/-/blob/master/LbDevTools/data/cmake/GaudiProjectConfig.cmake#L520-553.