fix Windows fixtures and Flatpak CI packaging

flatpak/build.sh

@@ -18,11 +18,14 @@ runtime_version=$(sed -n "s/^runtime-version: *'\(.*\)'/\1/p" "$id.yml")
 # --user so nothing here needs root, and --if-not-exists so a second run is free.
 flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
 flatpak install --user --noninteractive flathub \
+  org.flatpak.Builder \
   "org.gnome.Platform//$runtime_version" \
   "org.gnome.Sdk//$runtime_version"
 
 rm -rf build repo
-flatpak-builder --user --disable-rofiles-fuse --force-clean --repo=repo build "$id.yml"
+# Ubuntu 22.04's builder calls appstream-compose, which GNOME 48 no longer ships.
+# Use the Flathub builder for its current AppStream support on CI and local builds alike.
+flatpak run org.flatpak.Builder --user --disable-rofiles-fuse --force-clean --repo=repo build "$id.yml"
 flatpak build-bundle repo "$bundle" "$id"
 
 echo "Wrote $bundle"