nspawn: rework how /run/host/ is set up

Let's find the right os-release file on the host side, and only mount
the one that matters, i.e. /etc/os-release if it exists and
/usr/lib/os-release otherwise. Use the fixed path /run/host/os-release
for that.

Let's also mount /run/host as a bind mount on itself before we set up
/run/host, and let's mount it MS_RDONLY after we are done, so that it
remains immutable as a whole.