Add -Dscala.ext.dirs property for Java 9 compat

This reverts commit 1394ff019ec2337e99b007e338c87ecdbf266061.

Author: retronym

scripts/common

@@ -19,7 +19,7 @@ mkdir -p $IVY_CACHE
 rm -rf $IVY_CACHE/cache/org.scala-lang
 
 SBT_CMD=${sbtCmd-sbt}
-SBT_CMD="$SBT_CMD -sbt-version 0.13.14-b646662"
+SBT_CMD="$SBT_CMD -sbt-version 0.13.14-b646662 -Dscala.ext.dirs=$JAVA_HOME/scala-ext"
 
 # temp dir where all 'non-build' operation are performed
 TMP_ROOT_DIR=$(mktemp -d -t pr-scala.XXXX)