nix-tools: re‐add nixPackage

(With some tweaks to handle `nix.enable` and order it at a more
sensible position in the `$PATH`.)

The installers actually install Nix into `root`’s profile for some
reason, which means that the path’s prioritization backfires when
the script runs as root and we’re managing the Nix installation. When
running `darwin-rebuild` as a normal user, this wasn’t a problem.

Maybe we should just have a check to make sure there’s no conflicting
Nix in `root`’s profile – it seems pretty bad for `root` to
get the wrong Nix – but it would trigger for almost everyone,
which seems kind of annoying. I guess we could automatically
remove it from `root`’s profile if it matches what’s in
`/nix/var/nix/profiles/default`…

This reverts commit 02232f71c5.

modules/nix/nix-darwin.nix

@@ -5,6 +5,7 @@ let
     inherit (config.system) profile;
     inherit (config.environment) systemPath;
     nixPath = lib.optionalString config.nix.enable (lib.concatStringsSep ":" config.nix.nixPath);
+    nixPackage = if config.nix.enable then config.nix.package else null;
   };
 
   darwin-uninstaller = pkgs.callPackage ../../pkgs/darwin-uninstaller { };