Update flatpak-build.yml

.github/workflows/flatpak-build.yml

@@ -17,6 +17,7 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y flatpak flatpak-builder
+          flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
     
       - name: Checkout code
         uses: actions/checkout@v2