feature(settings): add cache, use more cores, auto optimize the nix store

modules/nixos/settings.nix

@@ -1,8 +1,36 @@
 { config, lib, pkgs, ... }:
 
 {
-  # use flakes and nix command
-  nix.settings.experimental-features = [ "nix-command" "flakes" ];
+  nix = {
+    # nix settings
+    settings = {
+      # enable flakes, nix-command and eval-cache
+      experimental-features = [ "nix-command" "flakes" "eval-cache" ];
+      # auto-optimize my nix-store
+      auto-optimise-store = true;
+      # use all cores
+      max-jobs = "auto";  
+      # use all available cores per job
+      cores = 0;          
+      # add trusted substituters (binary caches)
+      substituters = [
+        "https://cache.nixos.org"
+        "https://nix-community.cachix.org"
+      ];
+      # add keys
+      trusted-public-keys = [
+        "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      ];
+    };
+    
+    # enable automatic garbage collection
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
+  };
 
   # links /libexec from derivations to /run/current-system/sw
   environment.pathsToLink = [ "/libexec" ];