Make expectedHash optional in prim_path
Make expectedHash optional in prim_path

This fixes an error found in builtins.path that looks like:

store path mismatch in (possibly filtered) path added from '/private/tmp/nix-shell.CyXViH/nix-test/filter-source/filterin'

when no hash is specified

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