installer: ask to edit configuration before installing

This is skipped when stdin is not a tty, unlike the other interactive
parts that look at stdout.

pkgs/darwin-installer/default.nix

@@ -45,6 +45,17 @@ stdenv.mkDerivation {
       echo "copying example configuration.nix" >&2
       mkdir -p "$HOME/.nixpkgs"
       cp "${toString ../../modules/examples/simple.nix}" "$config"
+
+      # Skip when stdin is not a tty, eg.
+      # $ yes | darwin-installer
+      if test -t 0; then
+          read -p "Would you like edit the default configuration.nix before starting? [y/n] " i
+      fi
+      case "$i" in
+          e|E)
+              $EDITOR "$config"
+              ;;
+      esac
     fi
 
     export NIX_PATH=${nixPath}