This assumes that the root user is always uid 0 and gid 0, which I
believe is a safe assumption. The reason to add this is because when a
declarative VM (for example, a NixOS test) or image boots the first
time, the installRootOwnedSecrets activation script runs BEFORE the
"users" and "groups" activation scripts, so the user and group for
root is not created. Using uid 0 and gid 0 gets around the root user
not being set up yet.