nixUnstable: pre20211001 -> pre20211006
nixUnstable: pre20211001 -> pre20211006

The specific reason to update is to pull in logging fix:
  c6718a9d950214 "Don't reset the logger in a vfork"

Otherwise 'nix build' does not report build progress correctly.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs