Modify GitEntity resolver to support version override via environment variable
The name of the environment variable depends on the instance name, so for "ToolSvc.GitDDDB" it will be "DDDB_VERSION_OVERRIDE", while for "Dummy" it will be "Dummy_VERSION_OVERRIDE".
This is a port of the DetDesc part of LHCb!5170 (merged) and requires other changes that have been added there.