nixos/transmission: make it possible to use a different home directory (#29138)

nixos/modules/services/torrent/transmission.nix

···
   cfg = config.services.transmission;
   apparmor = config.security.apparmor.enable;
 
-  homeDir = "/var/lib/transmission";
+  homeDir = cfg.home;
   downloadDir = "${homeDir}/Downloads";
   incompleteDir = "${homeDir}/.incomplete";
 
···
         type = types.int;
         default = 9091;
         description = "TCP port number to run the RPC/web interface.";
+      };
+
+      home = mkOption {
+        type = types.path;
+        default = "/var/lib/transmission";
+        description = ''
+          The directory where transmission will create files.
+        '';
       };
     };
   };