Allow QA tests to test both current stable _and_ upcoming / beta versions of git and git-lfs tools

The git-lfs core folks kindly gave us the heads up that a new git-lfs version is coming out soon - https://github.com/git-lfs/git-lfs/pull/3520.

Let's come up with a way in which to automate the testing of both current stable upcoming / beta versions of git and git-lfs tools.