semaphoreci: Run subset of autopkgtests in LXC (#11814)

Run build/test in LXC for now, as full nested QEMU is too brittle right
now: https://github.com/semaphoreci/semaphore/issues/37
But this at least runs some tests. It ensures that systemd generally
works in containers, as well as provides some backup results if the main
Ubuntu CI is down.