systemd-nspawn: relax PrivateUsers check

this options allows two colon-separated number as value, for instances which
is quite difficult to check.

nixos/modules/system/boot/systemd-nspawn.nix

···
     ])
     (assertValueOneOf "Boot" boolValues)
     (assertValueOneOf "ProcessTwo" boolValues)
-    (assertValueOneOf "PrivateUsers" (boolValues ++ [ "pick" ]))
     (assertValueOneOf "NotifyReady" boolValues)
   ];