fix missing dx in build server.

Change-Id: I18c49c5cc1efc97199cb076dc258c4b9b7a46e8a
diff --git a/test/022-interface/build b/test/022-interface/build
index 36738c3..c86b1dc 100644
--- a/test/022-interface/build
+++ b/test/022-interface/build
@@ -19,5 +19,5 @@
 
 # Use classes that are compiled with ecj that exposes an invokeinterface
 # issue when interfaces override methods in Object
-dx --debug --dex --dump-to=classes.lst --output=classes.dex classes
+${DX} --debug --dex --dump-to=classes.lst --output=classes.dex classes
 zip $TEST_NAME.jar classes.dex