darwin-uninstaller: remove darwin channel from root too

pkgs/darwin-uninstaller/configuration.nix

@@ -16,12 +16,12 @@ with lib;
   nix.useDaemon = mkForce false;
 
   system.activationScripts.postUserActivation.text = mkAfter ''
-    if [[ -L ~/.nix-defexpr/channels/darwin ]]; then
-        nix-channel --remove darwin || true
-    fi
+    nix-channel --remove darwin || true
   '';
 
   system.activationScripts.postActivation.text = mkAfter ''
+    nix-channel --remove darwin || true
+
     if [[ -L /Applications/Nix\ Apps ]]; then
         rm /Applications/Nix\ Apps
     fi

pkgs/darwin-uninstaller/default.nix

@@ -77,7 +77,7 @@ in writeShellApplication {
       echo >&2
 
       echo >&2 "checking darwin channel"
-      test -e ~/.nix-defexpr/channels/darwin && exit 1
+      nix-instantiate --find-file darwin && exit 1
       echo >&2 "checking /etc"
       test -e /etc/static && exit 1
       echo >&2 "checking /run/current-system"