Avoid to fail with race condition

spec/bundler/support/builders.rb

@@ -421,7 +421,7 @@ module Spec
         build_path = @context.tmp + full_name
         bundler_path = build_path + "#{full_name}.gem"
 
-        Dir.mkdir build_path
+        Dir.mkdir build_path unless File.directory?(build_path)
 
         @context.shipped_files.each do |shipped_file|
           target_shipped_file = shipped_file