Use CLASSPATH env var to configure child permutation workers

This fixes a bug in windows where too long of arguments prevents the
process from being able to start.

Bug-Link: https://github.com/gwtproject/gwt/issues/9701
Bug: #9701
Change-Id: I572487e682ceae070878e76ae0dd29a771ef1dad

dev/core/src/com/google/gwt/dev/ExternalPermutationWorkerFactory.java

diff --git a/dev/core/src/com/google/gwt/dev/ExternalPermutationWorkerFactory.java b/dev/core/src/com/google/gwt/dev/ExternalPermutationWorkerFactory.java
index b6d4573..bcb56f5 100644
--- a/dev/core/src/com/google/gwt/dev/ExternalPermutationWorkerFactory.java
+++ b/dev/core/src/com/google/gwt/dev/ExternalPermutationWorkerFactory.java
@@ -261,8 +261,7 @@
     String cookie = StringUtils.toHexString(cookieBytes);
 
     // Cook up the classpath, main class, and extra args
-    args.addAll(Arrays.asList("-classpath",
-        ManagementFactory.getRuntimeMXBean().getClassPath(),
+    args.addAll(Arrays.asList(
         CompilePermsServer.class.getName(), "-host", "localhost", "-port",
         String.valueOf(port), "-logLevel", logLevel.toString(), "-cookie",
         cookie));
@@ -276,6 +275,7 @@
     }
 
     ProcessBuilder builder = new ProcessBuilder();
+    builder.environment().put("CLASSPATH", ManagementFactory.getRuntimeMXBean().getClassPath());
     builder.command(args);
 
     try {