LCG_88b Update, 21.6 branch (2020.09.16.)
This MR backports some of the build script updates that we made in the master branch a while ago to allow us setting the LCG version to use for the build, from this repository's code.
At the same time I've set up the AthGeneration
build to now use LCG_88b instead of LCG_88 as requested by @cgutscho.