Update Flatpak GitHub Actions image to gnome-49

Author: lainsce

.github/workflows/flatpak.yml

@@ -8,7 +8,7 @@ jobs:
     name: Flatpak
     runs-on: ubuntu-latest
     container:
-      image: bilelmoussaoui/flatpak-github-actions:gnome-43
+      image: bilelmoussaoui/flatpak-github-actions:gnome-49
       options: --privileged
     steps:
       - uses: actions/checkout@v2