fix: remove bogus bash warning when using actions.

bin/compose-core

@@ -3029,7 +3029,8 @@ charm.sanity_checks || die "Sanity checks about charm-store failed. Please corre
 if [ -z "$is_docker_compose_action" -a "$action" ]; then
     action_service=${remainder_args[0]}
     remainder_args=("${remainder_args[@]:1}")
-    if is_service_action=$(has_service_action "$action_service" "$action"); then
+    if has_service_action "$action_service" "$action" >/dev/null; then
+        is_service_action=true
         {
             read-0 action_type
             case "$action_type" in