deno: fix update script dependency

nix-prefetch-url comes with nix and isn't a package on it's own

pkgs/development/web/deno/update/update.ts

···
 #!/usr/bin/env nix-shell
 /*
-#!nix-shell -i "deno run --allow-net --allow-run --allow-read --allow-write" -p deno git nix-prefetch nix-prefetch-url
+#!nix-shell -i "deno run --allow-net --allow-run --allow-read --allow-write" -p deno git nix-prefetch
 */
 import {
   commit,