Use r8retrace.jar and not r8lib.jar for retracing

Change-Id: Ie2b9691c47adb5a9c670a9607b5de06329e424a7
diff --git a/tools/retrace.py b/tools/retrace.py
index 0aa5af2..14593ab 100755
--- a/tools/retrace.py
+++ b/tools/retrace.py
@@ -151,7 +151,7 @@
 
   retrace_args += [
     '-cp',
-    utils.R8_JAR if no_r8lib else utils.R8LIB_JAR,
+    utils.R8_JAR if no_r8lib else utils.R8RETRACE_JAR,
     'com.android.tools.r8.retrace.Retrace',
     map_path
   ]