Motivation for this change

The derivation for zfs currently doesn't pull in the same kernel if boot.kernel.randstructSeed is set to a non-zero length string resulting in an extra kernel to build while also basically destroying the purpose of setting a randstructSeed, since the kernel used to build zfs would use a different seed.

Using callPackage should also work since kernel won't be evaluated until the derivation is called, in which case the kernel would've been updated (I believe), but I'm not sure how that should be done without using a let/in block (which probably should be in all-packages.nix.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[1] https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix#L16933