[DEPS] switch download_nacl_toolchains.py hook back to python

download_nacl_toolchains.py became flaky after switching to vpython
and causing build breakages. Switch it back to python.

TBR=dpranke@chromium.org

Bug: 784251
Change-Id: I449bb25ff83254edabed249df0564cd0de3c8ec1
No-Try: true
No-Presubmit: true
Reviewed-on: https://chromium-review.googlesource.com/769327Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516370}

DEPS

@@ -735,7 +735,7 @@ hooks = [
     'pattern': '.',
     'condition': 'checkout_nacl',
     'action': [
-        'vpython',
+        'python',
         'src/build/download_nacl_toolchains.py',
         '--mode', 'nacl_core_sdk',
         'sync', '--extract',