dovecot: use marker option as unit name migration indicator

In nixpkgs we expose `services.dovecot.hasNewUnitName` option that can be
safely inspected to understand that whether to use the `dovecot` systemd
service name instead of `dovecot2`.

mail-server/nginx.nix

@@ -16,12 +16,20 @@
 
 {
   config,
+  options,
   pkgs,
   lib,
   ...
 }:
 
-with (import ./common.nix { inherit config lib pkgs; });
+with (import ./common.nix {
+  inherit
+    config
+    options
+    lib
+    pkgs
+    ;
+});
 
 let
   cfg = config.mailserver;
@@ -42,7 +50,7 @@ in
 
         security.acme.certs."${cfg.acmeCertificateName}".reloadServices = [
           "postfix.service"
-          "dovecot2.service"
+          "${dovecotUnitName}.service"
         ];
       };
 }