Have a look at this topic, or more specific the reply I linked to:
In short: This is due to a security update on GitHub and not Ubuntu specific, someone also mentioned Slackware on the IRC darktable channel. The above link shows one way of solving this, there are other, more global ways that might be of interest if you pull/clone more then darktable alone from GitHub via git. Do an internet search if you want/need to know more about his.