ci: set build to use 49 image

Author: hydroxycarbamide

.github/workflows/build.yml

@@ -31,7 +31,7 @@ jobs:
   flatpak:
     name: Flatpak
     container:
-      image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-48
+      image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-49
       options: --privileged
     strategy:
       # Don't fail the whole workflow if one architecture fails