Fold FCOS stream label injection into build-rootfs

[jlebon]

See

coreos-assembler/src/cmd-build-with-buildah

Lines 165 to 172 in e9034ee

	# XXX: Temporary hack until we have https://github.com/coreos/rpm-ostree/pull/5454
	# which would allow us to fold this back into the build process.
	# shellcheck source=/dev/null
	stream=$(yaml2json "$manifest" /dev/stdout | jq -r '.variables.stream')
	if [ "${stream}" != null ]; then
	set -- "$@" --label fedora-coreos.stream="$stream" \
	--annotation fedora-coreos.stream="$stream"
	fi

We have new rpm-ostree now, so we can do this.