hosts: add nix-cache module

hosts/eris/configuration.nix

@@ -10,10 +10,9 @@
       # Include the results of the hardware scan.
       ./hardware-configuration.nix
       inputs.sops-nix.nixosModules.sops
+      ../modules/nix-cache.nix
     ];
 
-  nix.settings.experimental-features = [ "nix-command" "flakes" ];
-
   boot.loader = {
     efi = {
       canTouchEfiVariables = true;
@@ -32,10 +31,8 @@
   time.timeZone = "Asia/Shanghai";
 
   nix.settings = {
+    experimental-features = [ "nix-command" "flakes" ];
     trusted-users = [ "origami" ];
-    substituters = [
-      "https://mirrors.ustc.edu.cn/nix-channels/store"
-    ];
     auto-optimise-store = true;
   };