I am not sure how to do this with Unix, but with Windows using Ghost we could the automate the installation of a fresh OS image and perform an installation (or, whatever operation). We had just about every MS OS flavor. When we were done, we would install another fresh OS image and start another test.
It seems like it would be possible to isolate a build machine that used a fresh OS image for each build. I am not sure about the implications for doing this in the Unix world.