The actions/checkout workflow does some heroics to try to fetch exactly
the commit being built and nothing else, even if asked to fetch the
history leading up to that commit. That means we don't end up having
enough information to get an accurate answer from "git describe".

Since we're intentionally relying on the git history here, we'll fetch
all of the tags explicitly after initial checkout. Although that does add
some delay to this step, we're intentionally doing this version
calculation only once as a separate workflow job so that all of the other
jobs can still benefit from this action's quicker checkout behavior.