ci: disable `allowAliases` in the pinned pkgs instance (#435597)

    else

      nixpkgs;

  pkgs = import nixpkgs' { inherit system; };

  pkgs = import nixpkgs' {

    inherit system;

    # Nixpkgs generally — and CI specifically — do not use aliases,

    # because we want to ensure they are not load-bearing.

    allowAliases = false;

  };

  fmt =

    let

        programs.keep-sorted.enable = true;

        # This uses nixfmt underneath,

        # the default formatter for Nix code.

        # This uses nixfmt underneath, the default formatter for Nix code.

        # See https://github.com/NixOS/nixfmt

        programs.nixfmt.enable = true;

        programs.nixfmt = {

          enable = true;

          package = pkgs.nixfmt;

        };

        programs.yamlfmt = {

          enable = true;