split nix --flake enabling out of modules/nix.nix into universal

modules/nix.nix

@@ -12,9 +12,9 @@ in
     };
   };
 
-  config = {
+  config = mkIf cfg.enable {
     # use our own binary cache
-    nix.settings = mkIf cfg.enable {
+    nix.settings = {
       substituters = [
         "https://nixcache.uninsane.org"
         "https://nix-community.cachix.org"
@@ -25,10 +25,5 @@ in
         "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
       ];
     };
-
-    # allow `nix flake ...` command
-    nix.extraOptions = ''
-      experimental-features = nix-command flakes
-    '';
   };
 }

modules/universal/default.nix

@@ -29,5 +29,10 @@
     # git claims it should use EDITOR, but it doesn't!
     GIT_EDITOR = "vim";
   };
+
+  # allow `nix flake ...` command
+  nix.extraOptions = ''
+    experimental-features = nix-command flakes
+  '';
 }