Install flatpak on the worker so we can use it for generating sources.

.github/workflows/update_manifest.yml

@@ -29,6 +29,10 @@ jobs:
         run: |
           pip install ruamel.yaml
 
+      - name: Install Flatpak
+        run: |
+          apt-get install -y flatpak
+
       - name: Check for Flatpak source updates via Flatpak External Data Checker
         uses: docker://ghcr.io/flathub/flatpak-external-data-checker:latest
         with:
@@ -45,7 +49,6 @@ jobs:
           [ -z "$(git status -s -uno)" ] || echo "updated=true" >> $GITHUB_OUTPUT
 
       - name: Generate dotnet sources
-        uses: docker://flatpak/flatpak-builder:base
         if: steps.is-updated.outputs.updated
         run: |
           ./generate-sources.py