Externalize shared python tools

Note: To be rabased after !607 (merged) and !606 (merged) are merged.

• change Gitlab-CI Docker image to latest LHCb one
• prefer host-binary-tag from PATH (see https://gitlab.cern.ch/lhcb-core/LbPlatformUtils/)
• use xenv from PATH or get it from the Git repository (see https://gitlab.cern.ch/gaudi/xenv)

Although having the scripts as part of Gaudi is desirable, it's also very useful to be able to update/fix them without having to make a new release of Gaudi.