ci: use REGISTRY_TOKEN for Forgejo container registry push

.github/workflows/nightly.yml

@@ -261,7 +261,7 @@ jobs:
       with:
         registry: git.huckle.dev
         username: ${{ github.actor }}
-        password: ${{ secrets.GITHUB_TOKEN }}
+        password: ${{ secrets.REGISTRY_TOKEN }}
 
     - name: Build and push image
       uses: https://github.com/docker/build-push-action@v6