Fix aborts when using builtins.getFlake
Fix aborts when using builtins.getFlake

In that case, 'self' could refer to a value on the stack, so accessing
'self.rev' would abort.

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