lib/default.nix: quelch security.acme.email warning on NixOS>21.05

lib/default.nix

@@ -2,7 +2,7 @@
 # That includes physical servers, VMs, containers, and personal machines.
 #
 
-{ config, lib, pkgs, ... }:
+{ config, options, lib, pkgs, ... }:
 
 let
   hqPrefix64 = "fd23:42:c3d2:523";
@@ -336,10 +336,18 @@ in {
     };
 
     # Defaults for LetsEncrypt
-    security.acme = {
-      acceptTerms = true;
-      email = cfg.acmeEmail;
-    };
+    security.acme =
+      if options.security.acme ? defaults
+      then {
+        acceptTerms = true;
+        # NixOS>=22.05
+        defaults.email = cfg.acmeEmail;
+      }
+      else {
+        acceptTerms = true;
+        # TODO: NixOS<=21.05
+        email = cfg.acmeEmail;
+      };
   };
 
   meta.maintainers = with lib.maintainers; [ ehmry ];