Disallow reading flake.lock
Disallow reading flake.lock

With --no-write-lock-file, it's possible that flake.lock is out of
sync with the actual inputs used by the evaluation. So doing fromJSON
(readFile ./flake.lock) will give wrong results.

Fixes #4639.

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