Change default to LbEnv
Currently this image defaults to LbLogin if no environment variables are set:
$ docker run --rm -it -v /cvmfs/:/cvmfs/ gitlab-registry.cern.ch/lhcb-core/lbdocker/centos7-build:latest
WARNING : using environment variable LHCBNIGHTLIES for nightlies_dir
********************************************************************************
* ---- LHCb Login v9r2p6 ---- *
* Building with gcc8 on centos7 x86_64 system (x86_64-centos7-gcc8-opt) *
********************************************************************************
--- LHCBPROJECTPATH is set to:
/cvmfs/lhcb.cern.ch/lib/lhcb
/cvmfs/lhcb.cern.ch/lib/lcg/releases
/cvmfs/lhcb.cern.ch/lib/lcg/app/releases
/cvmfs/lhcb.cern.ch/lib/lcg/external
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$ docker run -e LHCB_ENV_MODE=lbenv-stable --rm -it -v /cvmfs/:/cvmfs/ gitlab-registry.cern.ch/lhcb-core/lbdocker/centos7-build:latest
********************************************************************************
* ---- LbEnv ---- *
********************************************************************************
--- CMAKE_PREFIX_PATH is set to:
/cvmfs/lhcb.cern.ch/lib/lhcb
/cvmfs/lhcb.cern.ch/lib/lcg/releases
/cvmfs/lhcb.cern.ch/lib/lcg/app/releases
/cvmfs/lhcb.cern.ch/lib/lcg/external
/cvmfs/lhcb.cern.ch/lib/contrib
/cvmfs/lhcb.cern.ch/lib/var/lib/LbEnv/2129/stable/linux-64/lib/python3.8/site-packages/LbDevTools/data/cmake
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