Fix public key issues with apt (launchpad.net)

To add a public key to an Ubuntu repository, simply execute the following command and append the requested key (here 2EE5793634EF4A35 for rabbitVCS):

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 2EE5793634EF4A35

Version context

This procedure describes the software and the environment available at the time of its publication. The packages, interfaces, and repositories mentioned may have changed or disappeared; first check the system version and its support level.

cat /etc/os-release
uname -r

Work on a backup or a snapshot, note every modified file, and prepare for rollback. For a current workstation or server, prefer signed packages from officially supported repositories.

Reference: official Ubuntu lifecycle.