nixos/zfs: Fix regression that prevents people to boot from zfs using grub if they didn't add zfs to boot.initrd.supportedFilesystems

See https://github.com/NixOS/nixpkgs/pull/99386

nixos/modules/tasks/filesystems/zfs.nix

@@ -446,7 +446,8 @@ in
         '') rootPools));
       };
 
-      boot.loader.grub = mkIf inInitrd {
+      # TODO FIXME See https://github.com/NixOS/nixpkgs/pull/99386#issuecomment-798813567. To not break people's bootloader and as probably not everybody would read release notes that thoroughly add inSystem.
+      boot.loader.grub = mkIf (inInitrd || inSystem) {
         zfsSupport = true;
       };