Fix path to GN on Windows when running presubmit hooks

I think this fixes issues with `git cl upload` on Windows machines.
Note that fetch-gn grabs gn.exe (not gn.bat).

Change-Id: Iba64ec62cce00e8ba0076ce83d47e4e6e6ef6197
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271604
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed