syncthing: fix the application of allProxy

`systemd.user.services.<name>.Service.Environment` should be of type `(list of string) or string convertible to it`

Author: kiria-moe

modules/services/syncthing.nix

@@ -853,7 +853,7 @@ in
               3
               4
             ];
-            Environment = lib.mkIf (cfg.allProxy != null) { all_proxy = cfg.allProxy; };
+            Environment = lib.mkIf (cfg.allProxy != null) "all_proxy=${cfg.allProxy}";
 
             # Sandboxing.
             LockPersonality = true;