github fetcher: Don't use HEAD branch when cloning
github fetcher: Don't use HEAD branch when cloning

Fixes #4394.

NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix