Use python -m virtualenv on Windows and Linux

cibuildwheel/linux.py

@@ -89,7 +89,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                 # there are no dependencies that were pulled in at build time.
                 "$PYBIN/pip" install virtualenv
                 venv_dir=`mktemp -d`/venv
-                "$PYBIN/virtualenv" $venv_dir
+                "$PYBIN/python" -m virtualenv $venv_dir
                 source $venv_dir/bin/activate
 
                 # Check that we are using the Python from the virtual environment

cibuildwheel/windows.py

@@ -125,7 +125,7 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
         # there are no dependencies that were pulled in at build time.
         shell(['pip', 'install', 'virtualenv'], env=env)
         venv_dir = tempfile.mkdtemp()
-        shell(['virtualenv', venv_dir], env=env)
+        shell(['python', '-m', 'virtualenv', venv_dir], env=env)
         env['PATH'] = os.pathsep.join([os.path.join(venv_dir, 'Scripts'), env['PATH']])
 
         # check that we are using the Python from the virtual environment