child_process.fork: don't overwrite env

thanks to Malte-Thorben Bruns for pointing this out

lib/child_process_uv.js

@@ -140,7 +140,8 @@ exports.fork = function(modulePath, args, options) {
 
   // Just need to set this - child process won't actually use the fd.
   // For backwards compat - this can be changed to 'NODE_CHANNEL' before v0.6.
-  options.env = { NODE_CHANNEL_FD: 42 };
+  if (!options.env) options.env = { };
+  options.env.NODE_CHANNEL_FD = 42;
 
   // stdin is the IPC channel.
   options.stdinStream = createPipe(true);