Fix test trigger pipeline for 9.0 branch (#2465)

Author: breskeby

.buildkite/tests.trigger.sh

@@ -12,7 +12,7 @@ for BRANCH in "${BRANCHES[@]}"; do
     label: Trigger tests pipeline for $BRANCH
     async: true
     build:
-      branch: $BRANCH
+      branch: "$BRANCH"
 EOF
 done