[tools] Fix v8gen.py list command.

Bug: v8:11361
Change-Id: Ie36b612907fab01c269567e901494d2c7ea01b6d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2689192Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#72654}

tools/dev/v8gen.py

@@ -189,7 +189,7 @@ class GenerateGnArgs(object):
     return 0
 
   def cmd_list(self):
-    print('\n'.join(sorted(self._mbw.masters[self._options.master])))
+    print('\n'.join(sorted(self._mbw.builder_groups[self._options.master])))
     return 0
 
   def verbose_print_1(self, text):