Don't recreate existing presubmit venv.

Change-Id: If2a1aa52d67b8f1cd3dfe43fcf8a2d503c7575c8
diff --git a/presubmit.py b/presubmit.py
index ea5a0ff..3ad6a1c 100755
--- a/presubmit.py
+++ b/presubmit.py
@@ -33,7 +33,8 @@
 def _init_virtualenv():
     """Set up virtualenv, assumes recent Python 3 is already installed."""
     venv = os.path.abspath('.presubmit/venv')
-    call('python', '-m', 'venv', venv)
+    if not os.path.isdir(venv):
+        call('python', '-m', 'venv', venv)
     os.environ['PATH'] = os.pathsep.join((
         os.path.join(venv, 'bin'),
         os.environ['PATH'],