ninja windows: Use sys.executable rather than unqualified 'python'
ninja windows: Use sys.executable rather than unqualified 'python'

Isolate from the running environment, and make sure we're directly using the correct python binary. Also removes one cmd/c wrapper that's unnecessary since we're launching the binary directly.

R=thakis@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10656004

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp