Get rid of assumptions about ".nexe" extension.

tests/ppapi_example_post_message/nacl.scons

 # -*- python -*-
 # Copyright (c) 2011 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This is a Pepper PostMessage example from ppapi/examples/scripting.
 
 Import('env')
 
 env.Prepend(CPPDEFINES=['XP_UNIX'])
 
-nexe_name = 'ppapi_example_post_message_%s.nexe' % env.get('TARGET_FULLARCH')
+nexe_name = 'ppapi_example_post_message_${TARGET_FULLARCH}'
 nexe = env.ComponentProgram(
     nexe_name,
     ['$SOURCE_ROOT/ppapi/examples/scripting/post_message.cc'],
     EXTRA_LIBS=['${PPAPI_LIBS}', 'ppapi_cpp'])
 
 # Note that the html is required to run this program.
 env.Publish(nexe_name, 'run',
             ['ppapi_example_post_message.html',
              'ppapi_example_post_message.nmf'])
 
 
 test = env.PPAPIBrowserTester(
     'ppapi_example_post_message_test.out',
     url='ppapi_example_post_message.html',
     files=env.ExtractPublishedFiles(nexe_name),
     args=['--enable_experimental_js'])
 
 env.AddNodeToTestSuite(test,
                        ['chrome_browser_tests'],
                        'run_ppapi_example_post_message_test',
                        is_broken=env.PPAPIBrowserTesterIsBroken())