Environment: remove support for specifying :builders key in constructor

build_tests/custom_builder/build.rb

@@ -9,7 +9,8 @@ EOF
   end
 end
 
-Rscons::Environment.new(echo: :short, builders: [MySource.new]) do |env|
+Rscons::Environment.new(echo: :short) do |env|
+  env.add_builder(MySource.new)
   env.MySource('inc.h', [])
   env.Program('program', Dir['*.c'])
 end