nixos/octoprint: allow access to serial devices

Printers are usually connected over USB to serial interfaces that are
mounted as tty character devices owned by the dialout group. Add our
octoprint service to this group at runtime to allow access to these
printers.

nixos/modules/services/misc/octoprint.nix

@@ -122,6 +122,9 @@ in
         ExecStart = "${pluginsEnv}/bin/octoprint serve -b ${cfg.stateDir}";
         User = cfg.user;
         Group = cfg.group;
+        SupplementaryGroups = [
+          "dialout"
+        ];
       };
     };