If you don't care about Ubuntu "apport" on this system (something for sending crash/bug reports to developers with a GUI), you could uninstall it (otherwise it overwrites the core_pattern every time it restarts, no matter what you write in your sysctl.conf, apparently), and then sudo sysctl -w kernel.core_pattern=core to undo the setting immediately (or reboot). Then hopefully the build farm would succeed in dumping a backtrace into the log.
I forced-removed apport (which meant removing xserver-xorg). Let's see whether the reports are better or whether I break something.
-- Fabien.
