syncthing: create default group if not overridden

The following configuration generates a systemd unit that doesn't
start.
```nix
{
  services.syncthing = {
    enable = true;
    user = "my-user";
  };
}
```

It fails with
```
systemd[1]: Started Syncthing service.
systemd[6745]: syncthing.service: Failed to determine group credentials: No such process
systemd[6745]: syncthing.service: Failed at step GROUP spawning /nix/store/n1ydz3i08nqp1ajc50ycy1zribmphqc9-syncthing-1.1.4-bin/bin/syncthing: No such process
systemd[1]: syncthing.service: Main process exited, code=exited, status=216/GROUP
systemd[1]: syncthing.service: Failed with result 'exit-code'.
```

This is due to the fact that `syncthing` group (default) is not
created if the user is overridden.

Add a separate check for setting up the default group, so that
user/group are created independently.

1 files changed, 5 insertions, 3 deletions

diff --git a/nixos/modules/services/networking/syncthing.nix b/nixos/modules/services/networking/syncthing.nix
index 8148139c3a81..126f5b7b527b 100644
--- a/nixos/modules/services/networking/syncthing.nix
+++ b/nixos/modules/services/networking/syncthing.nix
@@ -372,16 +372,18 @@ in {
 
     systemd.packages = [ pkgs.syncthing ];
 
-    users = mkIf (cfg.systemService && cfg.user == defaultUser) {
-      users."${defaultUser}" =
+    users.users = mkIf (cfg.systemService && cfg.user == defaultUser) {
+      "${defaultUser}" =
         { group = cfg.group;
           home  = cfg.dataDir;
           createHome = true;
           uid = config.ids.uids.syncthing;
           description = "Syncthing daemon user";
         };
+    };
 
-      groups."${defaultUser}".gid =
+    users.groups = mkIf (cfg.systemService && cfg.group == defaultUser) {
+      "${defaultUser}".gid =
         config.ids.gids.syncthing;
     };