fix up announcing compose start

Signed-off-by: Dennis Gilmore <dennis@ausil.us>

nightly.sh

@@ -43,7 +43,7 @@ FEDMSG_CERTPREFIX="releng"
 . ./releng/scripts/fedmsg-functions.sh
 
 # Announce that we are starting, even though we don't know the compose_id yet..
-send_fedmsg $(printf '{"log": "start", "branch": "%s", "arch": "%s"}' "$BRANCHED" "$ARCH") ${DIST} start
+send_fedmsg "${fedmsg_json_start}" ${DIST} start
 
 pushd $TMPDIR
 git clone https://pagure.io/fedora-comps.git && {