remove some builder-tracked fields from the build_operation Hash

build_tests/typical/echo_command_string.rb

@@ -1,8 +1,7 @@
 class MyBuilder < Rscons::Builder
   def run(options)
-    env, target = options.values_at(:env, :target)
-    env.print_builder_run_message("MyBuilder #{target}", "MyBuilder #{target} command")
-    target
+    @env.print_builder_run_message("MyBuilder #{@target}", "MyBuilder #{@target} command")
+    @target
   end
 end