[ci] Fix Flatpak build

.github/workflows/ci.yml

@@ -29,10 +29,14 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - name: Install Xvfb
+      - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install -y xvfb
+          sudo apt-get install -y flatpak flatpak-builder xvfb
+
+      - name: Add Flathub repo
+        run: |
+          sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
 
       - name: Build Flatpak
         uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v6