Add a suffix to the version before build (at least if it's already released)

I have noticed that piuparts will fail quite often when the version to test is exactly the same than the version currently available in the mirror.

debian/changelog is often only updated as the last step in the work and as such our build step should at least try to increase the version by adding a small "+ci" or something like that. I don't know if we must do that in all cases or only if we figure out that the current version is actually already released... I guess the latter is best (maintainer scripts might have tests depending on the precise version) but at the same time it's not so great to introduce such variations based on a remote state.