Revert "nixosModules,pkgs: remove installer. clanModules: init installer module"

This reverts commit 29a7f0312b.

nixosModules/zfs.nix

@@ -0,0 +1,16 @@
+{ lib, config, ... }:
+{
+  # Use the same default hostID as the NixOS install ISO and nixos-anywhere.
+  # This allows us to import zfs pool without using a force import.
+  # ZFS has this as a safety mechanism for networked block storage (ISCSI), but
+  # in practice we found it causes more breakages like unbootable machines,
+  # while people using ZFS on ISCSI is quite rare.
+  networking.hostId = lib.mkDefault "8425e349";
+
+  services.zfs = lib.mkIf (config.boot.zfs.enabled) {
+    autoSnapshot.enable = true;
+    # defaults to 12, which is a bit much given how much data is written
+    autoSnapshot.monthly = lib.mkDefault 1;
+    autoScrub.enable = true;
+  };
+}