Updates to the upstream Onionspray repo do not prompt a rebuild

I ran into this issue while testing the upcoming v3: when updating the Onionspray version we want to use, the role's logic doesn't rebuild Onionspray. We probably want to allow the role's users to update Onionspray, and hence rebuild it, by just changing the onionspray_repo_version variable, or when rerunning the role if they don't specify a fixed revision.

It seems to me that we should register the ansible.builtin.git output, for example in a _onionspray_repo_git variable, then change this condition to:

not onionspray_check_has_been_built.stat.exists or _onionspray_repo_git.before != _onionspray_repo_git.after

Not sure how the upstream scripts behave when Onionspray is already built and we run the build script again: to be tested.