feat(cachix): add cachix configuration files and update host.nix

- Added `cachix.nix` for configuring Cachix substituters
- Added `cachix/nix-community.nix` for Nix Community Cachix settings
- Updated `lib/host.nix` to include `pkgs.nixosModules.cachixConfig` in the list of NixOS modules

cachix.nix

@@ -0,0 +1,13 @@
+
+# WARN: this file will get overwritten by $ cachix use <name>
+{ pkgs, lib, ... }:
+
+let
+  folder = ./cachix;
+  toImport = name: value: folder + ("/" + name);
+  filterCaches = key: value: value == "regular" && lib.hasSuffix ".nix" key;
+  imports = lib.mapAttrsToList toImport (lib.filterAttrs filterCaches (builtins.readDir folder));
+in {
+  inherit imports;
+  nix.settings.substituters = ["https://cache.nixos.org/"];
+}

cachix/nix-community.nix

@@ -0,0 +1,13 @@
+
+{
+  nix = {
+    settings = {
+      substituters = [
+        "https://nix-community.cachix.org"
+      ];
+      trusted-public-keys = [
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      ];
+    };
+  };
+}

lib/host.nix

@@ -42,6 +42,7 @@
 
         home-manager.nixosModules.home-manager
         ../hosts/${name}/default.nix
+        pkgs.nixosModules.cachixConfig
       ];
     };
 }