should export JAVA variable in the bin/cassandra and use that in the cassandra-env.sh when check for the java version

I forgot which jira we add this java -version check in the cassandra-env.sh (for adding jamm to the javaagent), but we should probably use the variable JAVA set in bin/cassandra (will need export) and use $JAVA instead of "java" in the cassandra-env.sh

In a situation where JAVA_HOME may have been properly set as the Sun's java but the PATH still have the OpenJDK's java in front, the check will fail to add the jamm.jar, even though the cassandra jvm is properly started via the Sun's java.