The ROS buildfarm recently changed its GPG signing key out of security concerns. This means that you have to add the new key to your trusted list, and remove the old one.
On both the robot and your workstation you need to add the new key:
sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
And remove the old one:
sudo apt-key del 421C365BD9FF1F717815A3895523BAEEB01FA116
More info here: