fix Flatpak tooling and AppImage graphics on modern Linux

.github/workflows/release.yml

@@ -261,8 +261,9 @@ jobs:
           ref: ${{ needs.prepare.outputs.tag }}
 
       - run: |
+          sudo add-apt-repository -y ppa:flatpak/stable
           sudo apt-get update
-          sudo apt-get install -y flatpak
+          sudo apt-get install -y flatpak flatpak-builder
 
       - name: Build the flatpak from the published deb
         env: