elisp packages: update-from-overlay.nix3.sh, a flakey thin wrapper

pkgs/applications/editors/emacs/elisp-packages/update-from-overlay.flake.sh

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+#!/usr/bin/env nix
+#! nix shell nixpkgs#bash nixpkgs#curl nixpkgs#nix nixpkgs#coreutils --command bash
+set -xeuo pipefail
+
+# Since flakes are not stable yet, we can't suppose everyone is using them.
+# Therefore, the original ./update-from-overlay should not be modified.
+# So, let's create a wrapper!
+
+source ./update-from-overlay