Update to build with official `e26` software stack

[chenel]

Currently the SL7 build uses a fragile setup where hand-built versions of some of the dependencies from CAFAnaCore live in my /exp/dune/app working area in the DUNE gpvms. This is a temporary workaround while official versions of those dependencies are being made and deployed to CVMFS. Once they exist, the build should be updated to use them.