Use "Environment" instead of "Rscons::Environment" in build tests

build_tests/cfile/Rsconscript

@@ -1,5 +1,5 @@
 build do
-  Rscons::Environment.new do |env|
+  Environment.new do |env|
     env.CFile("lexer.c", "lexer.l")
     env.CFile("parser.c", "parser.y")
   end

build_tests/cfile/error_unknown_extension.rb

@@ -1,5 +1,5 @@
 build do
-  Rscons::Environment.new do |env|
+  Environment.new do |env|
     env.CFile("file.c", "foo.bar")
   end
 end