pinwheel: Move installation of age to age module

hosts/pinwheel/modules/age/default.nix

@@ -1,3 +1,7 @@
+{ inputs, config, ... }:
+let
+  system = config.config-manager.system;
+in
 {
   age = {
     identityPaths = [
@@ -5,4 +9,8 @@
       "/home/alex/.ssh/alex.pinwheel"
     ];
   };
+
+  environment.systemPackages = [
+    inputs.agenix.packages."${system}".default
+  ];
 }