Configure nix settings (#257)

Only manage /etc/nix/nix.conf options based on nixpkgs options.
We cannot control nix-gc / nix-daemon services yet as they rely on users.

nix/modules/upstream/nixpkgs/default.nix

@@ -7,6 +7,7 @@
   imports =
     [
       ./nginx.nix
+      ./nix.nix
     ]
     ++
     # List of imported NixOS modules
@@ -15,6 +16,8 @@
       "/misc/meta.nix"
       "/security/acme/"
       "/services/web-servers/nginx/"
+      # nix settings
+      "/config/nix.nix"
     ];
 
   options =

nix/modules/upstream/nixpkgs/nix.nix

@@ -0,0 +1,25 @@
+{ lib, pkgs, ... }:
+{
+  options = {
+    # options coming from modules/services/system/nix-daemon.nix that we cannot import just yet because it
+    # depends on users. These are the minimum options we need to be able to configure Nix using system-manager.
+    nix = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = true;
+        description = ''
+          Whether to enable Nix.
+          Disabling Nix makes the system hard to modify and the Nix programs and configuration will not be made available by NixOS itself.
+        '';
+      };
+      package = lib.mkOption {
+        type = lib.types.package;
+        default = pkgs.nix;
+        defaultText = lib.literalExpression "pkgs.nix";
+        description = ''
+          This option specifies the Nix package instance to use throughout the system.
+        '';
+      };
+    };
+  };
+}