Improve FlakeRef::to_string()
Improve FlakeRef::to_string()

We were incorrectly using path syntax (i.e. /<ref>/<rev>) for Git
repositories. This is only valid for GitHub flakerefs.

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