ci: Fix ci issue

Author: lqiu96

.kokoro/nightly/downstream-protobuf-binary-compatibility.sh

@@ -50,7 +50,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
   popd
 
   # The `-s` argument filters the linkage check problems that stem from the artifact
-  program_args="-r --artifacts ${ARTIFACT_LIST},com.google.protobuf:protobuf-java:${PROTOBUF_RUNTIME_VERSION},com.google.protobuf:protobuf-java-util:${PROTOBUF_RUNTIME_VERSION} -s ${maven_coordinates}"
+  program_args="-r --artifacts ${ARTIFACT_LIST},com.google.protobuf:protobuf-java:${PROTOBUF_RUNTIME_VERSION},com.google.protobuf:protobuf-java-util:${PROTOBUF_RUNTIME_VERSION} -s ${ARTIFACT_LIST}"
   echo "Linkage Checker Program Arguments: ${program_args}"
   mvn -B -ntp exec:java -Dexec.mainClass="com.google.cloud.tools.opensource.classpath.LinkageCheckerMain" -Dexec.args="${program_args}"
 done