urbit: fix update script

pkgs/by-name/ur/urbit/update-bin.sh

···
 set -euo pipefail
 
 ROOT="$(dirname "$(readlink -f "$0")")"
-NIX_DRV="$ROOT/default.nix"
+NIX_DRV="$ROOT/package.nix"
 if [ ! -f "$NIX_DRV" ]; then
   echo "ERROR: cannot find urbit in $ROOT"
   exit 1