R 3.6.0 build script updated to use TeX Live 2019.
R 3.6.0 build script updated to use TeX Live 2019.

UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts