SITEROOT Setting Fixes, master branch (2021.01.27.)

Made it possible to use SITEROOT != "/afs/cern.ch" during the build. The CMake code was unfortunately not acknowledging the setting of that variable in the build environment before now. 😦

These are necessary to have the master--dev3LCG and master--dev4LCG nightlies "remember" where they should pick up data files from at runtime, without the user having to specify it himself/herself, as discussed at length in:

https://groups.cern.ch/group/atlas-comp-ASCIG/Lists/Archive/Flat.aspx?RootFolder=%2Fgroup%2Fatlas%2Dcomp%2DASCIG%2FLists%2FArchive%2Fa%20sym%2Dlink%20cvmfssft%2Ecern%2Echswlcg%20%28%2D%20%2Elcg%29%20is%20needed%20to%20replace%20afscern%2Echsw&FolderCTID=0x012002003B8F97C71259CF40BFBDAF9DB1662075

This should also allow us to fix ATLINFR-3946. (After some further tweaks in the Jenkins jobs.)