nixcache: only enable on moby

it's annoying to rebuild on desko/lappy, and have that fail when servo
is offline/unreachable.
and it's really silly to have servo use its own cache *over the
network*.

long-term would be better to do properly distributed builds instead of
the cache.

machines/moby/default.nix

@@ -32,6 +32,7 @@
     # pkgs.alacritty  # terminal; crashes phosh
   ];
 
+  colinsane.nixcache.enable = true;
   colinsane.gui.phosh.enable = true;
   boot.loader.grub.enable = false;
   mobile.bootloader.enable = false;

modules/default.nix

@@ -6,6 +6,7 @@
     ./hardware
     ./image.nix
     ./impermanence.nix
+    ./nix.nix
     ./services/duplicity.nix
     ./universal
   ];

modules/nix.nix

@@ -0,0 +1,34 @@
+{ lib, config, ... }:
+
+with lib;
+let
+  cfg = config.colinsane.nixcache;
+in
+{
+  options = {
+    colinsane.nixcache.enable = mkOption {
+      default = false;
+      type = types.bool;
+    };
+  };
+
+  config = {
+    # use our own binary cache
+    nix.settings = mkIf cfg.enable {
+      substituters = [
+        "https://nixcache.uninsane.org"
+        "https://nix-community.cachix.org"
+        "https://cache.nixos.org/"
+      ];
+      trusted-public-keys = [
+        "nixcache.uninsane.org:r3WILM6+QrkmsLgqVQcEdibFD7Q/4gyzD9dGT33GP70="
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      ];
+    };
+
+    # allow `nix flake ...` command
+    nix.extraOptions = ''
+      experimental-features = nix-command flakes
+    '';
+  };
+}

modules/universal/default.nix

@@ -4,7 +4,6 @@
   imports = [
     ./fs.nix
     ./home-manager.nix
-    ./nix.nix
     ./secrets.nix
     ./users.nix
     ./vpn.nix

modules/universal/nix.nix

@@ -1,21 +0,0 @@
-{ ... }:
-
-{
-  # use our own binary cache
-  nix.settings = {
-    substituters = [
-      "https://nixcache.uninsane.org"
-      "https://nix-community.cachix.org"
-      "https://cache.nixos.org/"
-    ];
-    trusted-public-keys = [
-      "nixcache.uninsane.org:r3WILM6+QrkmsLgqVQcEdibFD7Q/4gyzD9dGT33GP70="
-      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
-    ];
-  };
-
-  # allow `nix flake ...` command
-  nix.extraOptions = ''
-    experimental-features = nix-command flakes
-  '';
-}