ci: Run tests with Java 8

Author: lqiu96

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

@@ -29,13 +29,13 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
   pushd "$repo"
 
   # Compile with Java 11 and run the tests with Java 8
-  mvn clean compile -T 1C
+  mvn compile -T 1C
 
-  export JAVA_HOME="${JAVA8_HOME}"
-  export PATH=$JAVA_HOME/bin:$PATH
-
-  echo "The Java Version is"
-  java -version
+  if [ -n "${JAVA8_HOME}"  ]; then
+    surefire_opt="-Djvm=${JAVA8_HOME}/bin/java"
+  else
+    surefire_opt="-Djvm=${JAVA_HOME}/bin/java"
+  fi
 
   # Compile the Handwritten Library with the Protobuf-Java version to test source compatibility
   # Run unit tests to help check for any behavior differences (dependant on coverage)
@@ -48,6 +48,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
       -Denforcer.skip \
       -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
       -pl "${google_cloud_java_handwritten_maven_args}" -am \
+      "${surefire_opt}" \
       -T 1C
   else
     mvn test -B -V -ntp \
@@ -56,6 +57,7 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
       -Dmaven.javadoc.skip \
       -Denforcer.skip \
       -Dprotobuf.version=${PROTOBUF_RUNTIME_VERSION} \
+      "${surefire_opt}" \
       -T 1C
   fi