unify nix options

nixos/common/default.nix

@@ -1,8 +1,28 @@
-{ ... }: {
+{ lib, pkgs, ... }: {
   imports = [
     ./audio.nix
   ];
 
+  # use nix version 2.22
+  nix = {
+    package = pkgs.nixVersions.nix_2_22;
+
+    settings = {
+      auto-optimise-store = lib.mkDefault true;
+      experimental-features = [
+        "nix-command"
+        "flakes"
+        "ca-derivations"
+      ];
+    };
+
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than +3";
+    };
+  };
+
   # auto mount external drives
   services.devmon.enable = true;
   services.gvfs.enable = true;