recipes shouldn't build if a build was triggered by hand for the set PPA and nothing has changed

[Philip Muškovac]

https://code.launchpad.net/~neon/+recipe/project-neon-dbusmenu-qt/+build/13378

failed to upload with
INFO: Upload was rejected:
INFO: File project-neon-dbusmenu-qt_0+194-9~maverick1.tar.bz2 already exists in Project Neon, but uploaded version has different contents. See more information about this error in https://help.launchpad.net/Packaging/UploadErrors.

This is because I requested a build a few hours earlier and nothing changed since then. The Recipe should be able to remember that it already built this package for this PPA instead of trying to re-upload. And using {time} is something I would rather not do.