Pass SBT_OPTS environment through to sbt_launcher

sbt/sbt

@@ -25,4 +25,4 @@ fi
 export SPARK_HOME=$(cd "$(dirname $0)/.." 2>&1 >/dev/null ; pwd)
 export SPARK_TESTING=1  # To put test classes on classpath
 
-java -Xmx1200m -XX:MaxPermSize=350m -XX:ReservedCodeCacheSize=256m $EXTRA_ARGS -jar $SPARK_HOME/sbt/sbt-launch-*.jar "$@"
+java -Xmx1200m -XX:MaxPermSize=350m -XX:ReservedCodeCacheSize=256m $EXTRA_ARGS $SBT_OPTS -jar $SPARK_HOME/sbt/sbt-launch-*.jar "$@"