The symlink to /var/lib/nixos/machine-id never persists the ID,
systemd-machine-id-commit.service requires a writable /etc, and
machine_id_commit() does not follow symlinks for its mountpoint check.
So the backing file stays "uninitialized", every boot is
ConditionFirstBoot=yes, and the machine-id is random per boot.
Ship an empty regular file instead, systemd then overlays /run/machine-id
for the session, ConditionFirstBoot is correctly "no", commit is
cleanly condition-skipped, and the file is a usable bind target for
users that want persistence.
Fixes#523878
r-vdp
3eaafe4d48