Ensure compose jxl image format test runs in CI

[bbhtt]

We need Appstream 1.2.0 in CI but it is only available in Debian unstable and nowhere else.

flatpak-builder/tests/test-appstream.sh

Lines 71 to 88 in c2d9ea9

	# test compose jxl image format
	if appstream_has_version 1 2 0; then
	APPDIR=builddir_jxl \
	run_build \
	--mirror-screenshots-url=https://example.org/media \
	--state-dir .fp-compose-image-format-jxl \
	--compose-url-policy=full \
	--compose-image-format=jxl \
	org.flatpak.appstream_media.json
	gzip -cdq builddir_jxl/files/share/app-info/xmls/org.flatpak.appstream_media.xml.gz \
	> appstream.xml
	assert_file_has_content_extended appstream.xml '>https://example.org/media/org/flatpak/appstream_media/[^/]+/icons/128x128/org.flatpak.appstream_media.jxl</icon>'
	ok "compose jxl image format"
	else
	ok "# Skip AppStream < 1.2.0"
	fi