refactor nix settings

nix/os/snippets/nix-settings-holo-chain.nix

@@ -0,0 +1,16 @@
+{pkgs, ...}: {
+  nix.settings = {
+    substituters = [
+      "https://holochain-ci.cachix.org"
+      "https://holochain-ci-internal.cachix.org"
+      "https://cache.holo.host/"
+    ];
+
+    trusted-public-keys = [
+      "holochain-ci.cachix.org-1:5IUSkZc0aoRS53rfkvH9Kid40NpyjwCMCzwRTXy+QN8="
+      "holochain-ci-internal.cachix.org-1:QvVsSrTiearCjrLTVtNtJOdQCDTseXh7UXUuSMx46NE="
+      "cache.holo.host-1:lNXIXtJgS9Iuw4Cu6X0HINLu9sTfcjEntnrgwMQIMcE="
+      "cache.holo.host-2:ZJCkX3AUYZ8soxTLfTb60g+F3MkWD7hkH9y8CgqwhDQ="
+    ];
+  };
+}

nix/os/snippets/nix-settings.nix

@@ -0,0 +1,35 @@
+{
+  pkgs,
+  lib,
+  ...
+}: {
+  nix.daemonCPUSchedPolicy = "idle";
+  nix.daemonIOSchedClass = "idle";
+  nix.settings.max-jobs = lib.mkDefault "auto";
+  nix.settings.cores = lib.mkDefault 0;
+  nix.settings.sandbox = true;
+  nix.nixPath = [
+    "nixpkgs=${pkgs.path}"
+  ];
+
+  nix.experimental-features = [
+    "nix-command"
+    "flakes"
+    "ca-derivations"
+    "impure-derivations"
+  ];
+
+  nix.system-features = [
+    "recursive-nix"
+    "big-parallel"
+    "kvm"
+    "nixos-test"
+  ];
+
+  nix.registry = {
+    nixpkgs.to = {
+      type = "path";
+      path = pkgs.path;
+    };
+  };
+}