nixos/tlsrpt: reuse sender address as envelope sender

This prevents leaking the local user and hostname into the envelope,
which could prevent DKIM signing due to lack of keys for the MX hostname.

nixos/modules/services/mail/tlsrpt.nix

@@ -244,12 +244,12 @@ in
               type = with types; nullOr str;
               default =
                 if config.services.postfix.enable && config.services.postfix.setSendmail then
-                  "/run/wrappers/bin/sendmail -i -t"
+                  "/run/wrappers/bin/sendmail -i -t -f ${cfg.reportd.settings.sender_address}"
                 else
                   null;
               defaultText = lib.literalExpression ''
                 if config.services.postfix.enable && config.services.postfix.setSendmail then
-                  "/run/wrappers/bin/sendmail -i -t"
+                  "/run/wrappers/bin/sendmail -i -t -f $${cfg.reportd.settings.sender_address}"
                 else
                   null
               '';