Update to batch driver to use CMAKE env variables
In case the workdir is not set, trying to use the first part of the CMake path to provide the appropriate setup area for the event loop batch driver.
This is an attempt to respond to this help thread: https://groups.cern.ch/group/hn-atlas-PATHelp/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fhn%2Datlas%2DPATHelp%2FLists%2FArchive%2FWorkDir%5FDIR%20in%20AnalysisBase%2C22%2E2&FolderCTID=0x0120020084D7E80CB0C3394A8D54EF07C309B044