frebsd-update: fix --currently-running after r343122

PR:		234771
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Reported by:	Brandon Schneider
MFC with:	r343122
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18896

usr.sbin/freebsd-update/freebsd-update.sh

@@ -310,6 +310,7 @@ config_SourceRelease () {
 	if echo ${UNAME_r} | grep -qE '^[0-9.]+$'; then
 		UNAME_r="${UNAME_r}-RELEASE"
 	fi
+	export UNAME_r
 }
 
 # Define what happens to output of utilities