Fix cbuildbot_stages Archive unittest

BUG=None
TEST=Unittest

Change-Id: If5f15a17361b58a4bd03383c42b98511f007e5b5
Reviewed-on: http://gerrit.chromium.org/gerrit/4284
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
diff --git a/buildbot/cbuildbot_stages_unittest.py b/buildbot/cbuildbot_stages_unittest.py
index c31a853..4e20a44 100755
--- a/buildbot/cbuildbot_stages_unittest.py
+++ b/buildbot/cbuildbot_stages_unittest.py
@@ -850,7 +850,7 @@
     self.mox.StubOutWithMock(shutil, 'rmtree')
 
     self.options.buildbot = False
-    shutil.rmtree(mox.Regex(r'^%s' % self.build_root))
+    shutil.rmtree(mox.Regex(r'^%s' % self.build_root), ignore_errors=True)
     commands.LegacyArchiveBuild(
         self.build_root, self.bot_id, self._build_config,
         self.options.buildnumber, None, mox.Regex(r'^%s' % self.build_root),