nixos auto-upgrade: remove flag when flake

The `--no-build-output` flag that is added by default is only valid
for the old cli, which is not used when flakes are used.

Follow-up to c9daa81eff.

nixos/modules/tasks/auto-upgrade.nix

@@ -109,9 +109,8 @@ in {
       '';
     }];
 
-    system.autoUpgrade.flags = [ "--no-build-output" ]
-      ++ (if cfg.flake == null then
-        (if cfg.channel == null then
+    system.autoUpgrade.flags = (if cfg.flake == null then
+        [ "--no-build-output" ] ++ (if cfg.channel == null then
           [ "--upgrade" ]
         else [
           "-I"