nix-tools: set $NIX_PATH

This will be important once most users are running `sudo
darwin-rebuild` and therefore not getting their environment’s
`$NIX_PATH` passed through.

modules/nix/nix-darwin.nix

@@ -4,6 +4,7 @@ let
   nix-tools = pkgs.callPackage ../../pkgs/nix-tools {
     inherit (config.system) profile;
     inherit (config.environment) systemPath;
+    nixPath = lib.concatStringsSep ":" config.nix.nixPath;
   };
 
   darwin-uninstaller = pkgs.callPackage ../../pkgs/darwin-uninstaller { };

pkgs/nix-tools/darwin-option.sh

@@ -1,7 +1,9 @@
 #! @shell@
 set -e
 set -o pipefail
+
 export PATH=@path@
+export NIX_PATH=${NIX_PATH:-@nixPath@}
 
 evalNix() {
   nix-instantiate --eval --strict "${extraEvalFlags[@]}" -E "with import <darwin> {}; $*" 2>/dev/null

pkgs/nix-tools/darwin-rebuild.sh

@@ -1,7 +1,9 @@
 #! @shell@
 set -e
 set -o pipefail
+
 export PATH=@path@
+export NIX_PATH=${NIX_PATH:-@nixPath@}
 
 showSyntax() {
   echo "darwin-rebuild [--help] {edit | switch | activate | build | check | changelog}" >&2

pkgs/nix-tools/default.nix

@@ -21,6 +21,11 @@
   "/usr/sbin"
   "/sbin"
 ]
+, # This should be kept in sync with the default `nix.nixPath`.
+  nixPath ? lib.concatStringsSep ":" [
+  "darwin-config=/etc/nix-darwin/configuration.nix"
+  "/nix/var/nix/profiles/per-user/root/channels"
+]
 }:
 
 let
@@ -39,14 +44,14 @@ in
 {
   darwin-option = writeProgram "darwin-option"
     {
-      inherit path;
+      inherit path nixPath;
       inherit (stdenv) shell;
     }
     ./darwin-option.sh;
 
   darwin-rebuild = writeProgram "darwin-rebuild"
     {
-      inherit path profile;
+      inherit path nixPath profile;
       inherit (stdenv) shell;
       postInstall = ''
         mkdir -p $out/share/zsh/site-functions