pinwheel/manatee/backwards: Adjust for updated nixpkgs

hosts/pinwheel/modules/age/default.nix

@@ -11,7 +11,7 @@
     };
 
     environment.systemPackages = [
-      inputs.agenix.packages."${pkgs.system}".default
+      inputs.agenix.packages."${pkgs.stdenv.hostPlatform.system}".default
     ];
   };
 }