Multiarch builds (lxd) always time out on failure instead of exit

Don’t set errexit in the main shell. This shell also runs the build’s internal logic and the sudden exit terminates it abnormally.

If a failing command should terminate the build, put it into a section other than script: – i.e. install. The script: section is designed to run tests – i.e. stuff that you want to run to completion even if something fails.