[nix] Make own module

nixos/modules/nix/default.nix

@@ -0,0 +1,6 @@
+{
+  imports = [
+    ./flakes
+    ./gc
+  ];
+}

nixos/modules/nix/flakes/default.nix

@@ -0,0 +1,14 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.sisyphus.nix.flakes;
+in {
+  options.sisyphus.nix.flakes.enable = lib.mkEnableOption "Nix Flakes";
+
+  config.nix = lib.mkIf cfg.enable {
+    extraOptions = ''
+      experimental-features = nix-command flakes
+    '';
+    package = pkgs.nixFlakes;
+  };
+}

nixos/modules/nix/gc/default.nix

@@ -0,0 +1,25 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.sisyphus.nix.gc;
+in {
+  options.sisyphus.nix.gc = {
+    weekly.enable = lib.mkEnableOption "Scheduled Nix garbage-collection";
+    onFull.enable = lib.mkEnableOption "Nix garbage-collection when disk is almost full";
+  };
+
+  config.nix = {
+    # If the disk has less than 100MiB, free up to 2GiB by garbage-collecting.
+    extraOptions = lib.mkIf cfg.onFull.enable ''
+      min-free = ${toString (100 * 1024 * 1024)}
+      max-free = ${toString (2048 * 1024 * 1024)}
+    '';
+
+    # Scheduled garbage-collect
+    gc = lib.mkIf cfg.weekly.enable {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 30d";
+    };
+  };
+}