Install flatpak and builder

.github/workflows/flatpak.yml

@@ -21,6 +21,13 @@ jobs:
         with:
           submodules: true
           fetch-depth: 0
+      - name: Install Flatpak tooling
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y flatpak flatpak-builder xvfb
+      - name: Add Flathub remote
+        run: |
+          flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
       - name: Get version
         id: get_version
         run: |