--- a/testing/mozharness/mozharness/base/python.py
+++ b/testing/mozharness/mozharness/base/python.py
@@ -415,30 +415,24 @@ class VirtualenvMixin(object):
             for module in ('distribute', 'pip'):
                 if c.get('%s_url' % module):
                     self.download_file(c['%s_url' % module],
                                        parent_dir=dirs['abs_work_dir'])
 
             virtualenv_options = c.get('virtualenv_options',
                                        ['--no-site-packages', '--distribute'])
 
-            # ugly hack to avoid virtualenv 15.1.0 hitting the network.
-            # taskcluster uses virtualenv 15.1.0; buildbot has been
-            # using virtualenv 1.7.1.2
-            # https://github.com/pypa/setuptools/issues/1042
-            if os.environ.get("TASK_ID"):
-                virtualenv_options.append("--no-download")
-
         if os.path.exists(self.query_python_path()):
             self.info("Virtualenv %s appears to already exist; skipping virtualenv creation." % self.query_python_path())
         else:
             self.mkdir_p(dirs['abs_work_dir'])
             self.run_command(virtualenv + virtualenv_options + [venv_path],
                              cwd=dirs['abs_work_dir'],
                              error_list=VirtualenvErrorList,
+                             partial_env={'VIRTUALENV_NO_DOWNLOAD': "1"},
                              halt_on_failure=True)
 
         # Resolve the pip version so we can conditionally do things if we have
         # a modern pip.
         pip = self.query_python_path('pip')
         output = self.get_output_from_command([pip, '--version'],
                                               halt_on_failure=True)
         words = output.split()