fix: Use modified update-flake-lock action

It uses an updated create-pull-request action that works with Gitea/Forgejo

.github/workflows/update.yml

@@ -17,7 +17,7 @@ jobs:
         with:
           fail-mode: true
       - name: Update flake.lock
-        uses: https://github.com/DeterminateSystems/update-flake-lock@main
+        uses: https://forgejo.stefka.eu/jiriks74/update-flake-lock@main
         with:
           # inputs: dzgui dzgui-testing # We'll see whether I want to limit the updates to dzgui
           token: ${{ secrets.FORGEJO_TOKEN_FOR_UPDATES }}