nixos/nix-daemon: Enable cgroups delegation (#339310)

When `use-cgroups` is enabled, the nix daemon creates sub-cgroups for the build processes (and itself if NixOS/nix#11412 is merged, see NixOS/nix#9675). `Delegate` should be set to prevent systemd from messing with the nix service's cgroups (https://github.com/systemd/systemd/blob/main/docs/CGROUP_DELEGATION.md) and ensure the OOM killer only targets the offending derivation and not the entire service (NixOS/nix#10374).

nixos/modules/services/system/nix-daemon.nix

@@ -198,6 +198,7 @@ in
             IOSchedulingClass = cfg.daemonIOSchedClass;
             IOSchedulingPriority = cfg.daemonIOSchedPriority;
             LimitNOFILE = 1048576;
+            Delegate = "yes";
           };
 
         restartTriggers = [ config.environment.etc."nix/nix.conf".source ];