home: add nix

This is basically the same thing as the NixOS module, except for
NIX_PATH handling [1].

[1]: https://github.com/nix-community/home-manager/pull/2677

home/default.nix

@@ -19,6 +19,7 @@
     ./jq
     ./mail
     ./mpv
+    ./nix
     ./nix-index
     ./nm-applet
     ./packages

home/nix/default.nix

@@ -0,0 +1,39 @@
+# Nix related settings
+{ config, inputs, lib, options, pkgs, ... }:
+let
+  cfg = config.my.home.nix;
+in
+{
+  options.my.home.nix = with lib; {
+    enable = my.mkDisableOption "nix configuration";
+
+    addToRegistry = my.mkDisableOption "add inputs and self to registry";
+
+    overrideNixpkgs = my.mkDisableOption "point nixpkgs to pinned system version";
+  };
+
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      nix = {
+        package = lib.mkDefault pkgs.nix;
+
+        settings = {
+          experimental-features = [ "nix-command" "flakes" ];
+        };
+      };
+    }
+
+    (lib.mkIf cfg.addToRegistry {
+      nix.registry = {
+        # Allow me to use my custom package using `nix run self#pkg`
+        self.flake = inputs.self;
+        # Use pinned nixpkgs when using `nix run pkgs#<whatever>`
+        pkgs.flake = inputs.nixpkgs;
+        # ... And also with `nix run nixpkgs#<whatever>`
+        nixpkgs.flake = lib.mkIf cfg.overrideNixpkgs inputs.nixpkgs;
+        # Add NUR to run some packages that are only present there
+        nur.flake = inputs.nur;
+      };
+    })
+  ]);
+}