home: nix: add 'NIX_PATH' handling

Still waiting for the actual home-manager module option to be added [1].

In the meantime, just do it by hand.

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

home/nix/default.nix

@@ -26,10 +26,24 @@ in
 
     addToRegistry = my.mkDisableOption "add inputs and self to registry";
 
+    addToNixPath = my.mkDisableOption "add inputs and self to nix path";
+
     overrideNixpkgs = my.mkDisableOption "point nixpkgs to pinned system version";
   };
 
   config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      assertions = [
+        {
+          assertion = cfg.addToNixPath -> cfg.linkInputs;
+          message = ''
+            enabling `my.home.nix.addToNixPath` needs to have
+            `my.home.nix.linkInputs = true`
+          '';
+        }
+      ];
+    }
+
     {
       nix = {
         package = lib.mkDefault pkgs.nix; # NixOS module sets it unconditionally
@@ -60,5 +74,9 @@ in
         in
         makeLinks channels;
     })
+
+    (lib.mkIf cfg.addToNixPath {
+      home.sessionVariables.NIX_PATH = "${config.xdg.configHome}/nix/inputs\${NIX_PATH:+:$NIX_PATH}";
+    })
   ]);
 }