Clang package.py: invoke gsutil.py with python explicitly
Clang package.py: invoke gsutil.py with python explicitly

Just invoking "gsutil.py" on its own doesn't work on Windows.

Also use ' '.join() instead of string.join() to avoid having
to import the string module.

BUG=578306

Review URL: https://codereview.chromium.org/1715053002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376592}

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