Disable convert_to_istanbul_test.py on Windows

Change crrev.com/c/4195841 altered this so that it uses Node which
doesn't work on Windows which then caused presubmit failures. This
disables the test on Windows to avoid this problem.

This was found by the Windows presubmit --all builder, in this build:

https://ci.chromium.org/ui/p/chromium/builders/ci/win-presubmit/178/overview

The error is most easily reproduced (on Windows) with this command:

  git cl presubmit --force --files "testing/merge_scripts/*"

Change-Id: I6d1b9f8c1e39e2056351e49ca1ee0c9d7055db86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4219064
Reviewed-by: Ben Reich <benreich@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1100673}