git: Add flathub repository to flatpak CI

.github/workflows/build.yml

@@ -1151,6 +1151,7 @@ jobs:
       - name: ⬇️ Install setup dependencies
         run: |
           sudo apt update && sudo apt install -y git curl flatpak-builder
+          flatpak remote-add --if-not-exists --user flathub https://dl.flathub.org/repo/flathub.flatpakrepo
 
       - name: 🧰 Checkout
         uses: actions/checkout@v4