It was disabled in c6953d1ff6 because
a recent Nixpkgs bump brought in a new systemd which changed how
systemd-nspawn worked.
As far as I can tell, the issue was caused by this upstream systemd
commit:
b71a0192c0
Bind-mounting the host's `/sys` and `/proc` into the container's
`/run/host/{sys,proc}` fixes the issue and allows the test to succeed.
This is broken because of a change in systemd in NixOS 23.05. It fails
with
Failed to mount proc (type proc) on /proc (MS_NOSUID|MS_NODEV|MS_NOEXEC ""): Operation not permitted
