Retry: fix the Python invocation for cros_chromite
Retry: fix the Python invocation for cros_chromite

Partial reimplementation of reverted
https://codereview.chromium.org/890803003/

This fixes the Python invocation part. The runhooks invocation
(recursion) part that caused the other CL to be reverted will be
addressed elsewhere.

BUG=414436
TEST=local
  - Ran `gclient sync`, works.
  - Ran `gclient runhooks`, works.

Review URL: https://codereview.chromium.org/888113002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293885 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build