Add path attribute to root default.nix

This makes it possible to refer to the path of Home Manager when you
just have a Nix expression, not the actual source. Some things run
import on a source and just give access to the result of the import,
not the source.

PR #1259

default.nix

@@ -7,4 +7,6 @@ rec {
     pkgs.callPackage ./home-manager/install.nix { inherit home-manager; };
 
   nixos = import ./nixos;
+
+  path = ./.;
 }