nixos-upgrade does not have gzip in $PATH

configuration.nix

@@ -7,7 +7,7 @@ in {
     # check https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/tags
     # for new releases
     (builtins.fetchTarball {
-      url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/v2.2.1/nixos-mailserver-v2.2.1.tar.gz";
+      url = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/archive/v2.2.1/nixos-mailserver-v2.2.1.tar";
       sha256 = "03d49v8qnid9g9rha0wg2z6vic06mhp0b049s3whccn1axvs2zzx";
     })
   ];