core: copy the host's os-release for /run/host/os-release

core: copy the host's os-release for /run/host/os-release

Currently for portable services we automatically add a bind mount
os-release -> /run/host/os-release. This becomes problematic for the
soft-reboot case, as it's likely that portable services will be configured
to survive it, and thus would forever keep a reference to the old host's
os-release, which would be a problem because it becomes outdated, and also
it stops the old rootfs from being garbage collected.

Create a copy when the manager starts under /run/systemd/propagate instead,
and bind mount that for all services using RootDirectory=/RootImage=, so
that on soft-reboot the content gets updated (without creating a new file,
so the existing bind mounts will see the new content too).

This expands the /run/host/os-release protocol to more services, but I
think that's a nice thing to have too.

Closes https://github.com/systemd/systemd/issues/28023