fix(nixos/sys): Switch Linux kernel from latest ZFS supported version…

… to latest LTS kernel

`zfs.package.latestCompatibleLinuxPackages` was recently bumped to 5.18
[0] in nixpkgs nixos-22.05, however that kernel release includes a
change [1] that causes ZFS to fail to compile [2] [3]. To workaround
this, downgrade the kernel to 5.15, until an OpenZFS release that
includes the bug fix [4] is released.

[0]: NixOS/nixpkgs#178830
[1]: https://patchwork.kernel.org/project/linux-kbuild/patch/[email protected]/
[2]: https://github.com/PetarKirov/dotfiles/runs/7166609186?check_suite_focus=true
[3]: openzfs/zfs#13528
[4]: openzfs/zfs@ff7e405

nixos/sys/boot.nix

@@ -3,7 +3,7 @@
   pkgs,
   ...
 }: {
-  boot.kernelPackages = pkgs.lib.mkOverride 1 config.boot.zfs.package.latestCompatibleLinuxPackages;
+  boot.kernelPackages = pkgs.lib.mkOverride 1 linuxKernel.packages.linux_5_15;
   boot.kernel.sysctl."kernel.perf_event_paranoid" = 1;
   boot.supportedFilesystems = ["zfs"];
   boot.zfs.forceImportRoot = false;