Revert Acts MaterialSource flag back to None temporarily
!59726 (merged) switched to loading ACTS material maps from CVMFS. We don't currently have a map for 01-01-00 deployed though, which breaks the Run4 full chain ART test. Since we can't update CVMFS files right now, this MR reverts to None which should fix the test.
We should revert this as soon as we can update CVMFS.
Ideally we would like to get this merged before today's nightly, so the ART test can run through.