update nix config

global/system-configuration.nix

@@ -30,7 +30,25 @@
             "nix-command"
             "flakes"
         ];
-    };
-    nix.gc.automatic = true;
 
+        substituters = [ 
+            "https://mirrors.ustc.edu.cn/nix-channels/store"            # 中科大
+            "https://mirrors.tuna.tsinghua.edu.cn/nix-channels/store"   # 清华
+            "https://mirrors.bfsu.edu.cn/nix-channels/store"            # 北外
+            "https://mirror.sjtu.edu.cn/nix-channels/store"             # 交大
+            "https://nixos-cn.cachix.org"
+            "https://cache.nixos.org/"
+        ];
+
+        trusted-users = [
+            "root"
+            "@wheel"
+        ];
+    };
+
+    nix.gc = {
+        automatic = true;
+        dates = "weekly";
+        options = "--delete-older-than 1w";
+    };
 }