adding modularity

utils/binary-cache.nix

@@ -0,0 +1,22 @@
+{
+  lib,
+  ...
+}: {
+
+  # ...
+  nix.settings = {
+    # allow substituters to be used
+
+    substituters = [
+        # also add mirrors?
+        #   "https://mirrors.ustc.edu.cn/nix-channels/store"
+        "https://cache.nixos.org"
+    ];
+
+    trusted-public-keys = [
+      # the default public key of cache.nixos.org, it's built-in, no need to add it here
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+    ];
+  };
+
+}