Before this change, systemd-oomd startup was flaky at least with
either systemd-sysusers or userborn enabled. It would restart several
times until users were provisioned, so that it finally succeeded.
An alternative would be to use a DynamicUser which was my first
approach, before I discovered that upstream added the after statement
in Dec 2024[1]. DynamicUsers could have further
implications (sandboxing, etc), so we follow upstream here.
It's not clear to me we why Upstreams "After=systemd-sysusers.service"
doesn't show up on nixos-unstable systems (systemd v257.6).
Userborn is covered, as its unit is aliased to systemd-sysusers.service.
The following test succeeded after this change on x86_64-linux:
nix-build -A nixosTests.systemd-oomd
[1]: 36dd429680
dd88db29ed