ci: switch to Forgejos upload artifact v4 fork to make it work

https://code.forgejo.org/forgejo/upload-artifact/src/tag/v4

.forgejo/workflows/deploy.yaml

@@ -73,7 +73,7 @@ jobs:
         run: |
           rsync -v -r --delete -e "ssh -i deploykey.priv -o 'UserKnownHostsFile ./known_hosts'" public/ ht-ccchh-website-deploy@public-web-static-intern.hamburg.ccc.de:/var/www/hackertours.hamburg.ccc.de/
 
-      - uses: actions/upload-artifact@v4
+      - uses: https://code.forgejo.org/forgejo/upload-artifact@v4
         if: github.event_name == 'pull_request'
         with:
           name: website-build