Can we provide instructions on the Wiki on how to import a pinned nixpkgs tarball boot.kernelPackages? This works for me:
boot.kernelPackages = let
pinnedNixPkgs = import (pkgs.fetchFromGitHub {
owner = "nixos";
repo = "nixpkgs";
rev = "35955e360c6851e9a53fed945a9e98cfea5d67be";
hash = "sha256-M4ml2ECaO7UBKsMhtw3ZOMFE/4y51W84xoFlmsb2sHw=";
}) {};
myKernel = pinnedNixPkgs.linux_5_19;
in pkgs.recurseIntoAttrs (pinnedNixPkgs.linuxPackagesFor myKernel);