How do you do it? I literally remove the flake from my nix profile and re-add it with a specific commit hash. I had troubles that just specifying master didn't update to the current master.
and then I added this blurb to my Nix config:
nixpkgs.config = { allowUnfree = true; packageOverrides = pkgs: { unstable = import <nixos-unstable> { config = config.nixpkgs.config; }; }; };