Fix gitea

Fixes this error:

> Migration Error: Your database (migration version: 299) is for a newer
Gitea, you can not use the newer database for this old Gitea release
(280).

This was probably caused by installing Gitea whilst experimenting again
with nixpkgs unstable.

modules/nixos/desktop/gitea.nix

@@ -1,6 +1,9 @@
+{ pkgs, ... }:
+
 {
   services.gitea = {
     enable = true;
+    package = pkgs.unstable.gitea;
 
     settings = {
       server = {