nixos/mailman: add package option

nixos/modules/services/mail/mailman.nix

@@ -9,7 +9,7 @@ let
   cfg = config.services.mailman;
 
   inherit
-    (pkgs.mailmanPackages.buildEnvs {
+    (cfg.packageSet.buildEnvs {
       withHyperkitty = cfg.hyperkitty.enable;
       withLDAP = cfg.ldap.enable;
     })
@@ -117,6 +117,10 @@ in
         description = "Enable Mailman on this host. Requires an active MTA on the host (e.g. Postfix).";
       };
 
+      packageSet = lib.mkPackageOption pkgs "mailmanPackages" { } // {
+        type = lib.types.attrs;
+      };
+
       ldap = {
         enable = lib.mkEnableOption "LDAP auth";
         serverUri = lib.mkOption {
@@ -348,7 +352,7 @@ in
       mailman.layout = "fhs";
 
       "paths.fhs" = {
-        bin_dir = "${pkgs.mailmanPackages.mailman}/bin";
+        bin_dir = "${cfg.packageSet.mailman}/bin";
         var_dir = "/var/lib/mailman";
         queue_dir = "$var_dir/queue";
         template_dir = "$var_dir/templates";