Add update script

This script runs nix-update for custom packages and updates the
flake.lock file.

For some reason, nix-update doesn't use passthru.updateScript on a flake
package.

update.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p nix-update
+
+set -Eeuxo pipefail
+
+cd "$(dirname "$0")"
+
+nix flake update
+
+nix-update --flake redlib --version=branch=main