nixos/logind: add "sleep" option to logindHandlerType

The "systemctl sleep" command was added in systemd v256.

Author: SebastianStork

nixos/modules/system/boot/systemd/logind.nix

@@ -18,6 +18,7 @@ let
     "hibernate"
     "hybrid-sleep"
     "suspend-then-hibernate"
+    "sleep"
     "lock"
   ];
 in