electrs/update.sh: ensure tag is checked out
electrs/update.sh: ensure tag is checked out

Cloning a tag-named branch introduced a supply chain attack vector,
because branch and tag contents might differ.
Now the hashed worktree always corresponds to the tag that is GPG-verified.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs