nixos/autoUpgrade: add runGarbageCollection option (#441565)

nixos/modules/tasks/auto-upgrade.nix

@@ -184,6 +184,15 @@ in
         '';
       };
 
+      runGarbageCollection = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        description = ''
+          Whether to automatically run `nix-gc.service` after a successful
+          system upgrade.
+        '';
+      };
+
     };
 
   };
@@ -197,6 +206,12 @@ in
           The options 'system.autoUpgrade.channel' and 'system.autoUpgrade.flake' cannot both be set.
         '';
       }
+      {
+        assertion = (cfg.runGarbageCollection -> config.nix.enable);
+        message = ''
+          The option 'system.autoUpgrade.runGarbageCollection = true' requires 'nix.enable = true'.
+        '';
+      }
     ];
 
     system.autoUpgrade.flags = (
@@ -218,6 +233,9 @@ in
 
       restartIfChanged = false;
       unitConfig.X-StopOnRemoval = false;
+      unitConfig.OnSuccess = lib.optional (
+        cfg.runGarbageCollection && config.nix.enable
+      ) "nix-gc.service";
 
       serviceConfig.Type = "oneshot";