use python3 for download_from_google_storage.bat

This is to fix
https://ci.chromium.org/ui/p/chromium/builders/ci/Win%20x64%20Builder%20(py2%20less)/1151/overview

Bug: 1327534
Change-Id: I3dcead03d0f4e213e1c4e610d8b90fe95ea843d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3670999
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>

download_from_google_storage.bat

@@ -9,4 +9,4 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python "%~dp0\download_from_google_storage.py" %*
+python3 "%~dp0\download_from_google_storage.py" %*