correctly report the exit value

Takashi Matsuo · Takashi Matsuo · commit c01b34a0d68c · 2020-05-28T22:43:48.000Z
diff --git a/.kokoro/trampoline_v2.sh b/.kokoro/trampoline_v2.sh
@@ -357,3 +357,5 @@ if [[ "${update_cache}" == "true" ]] && \
 	log_red "Failed uploading the Docker image."
     fi
 fi
+
+exit "${test_retval}"
