Work around python 2 shutil.copy2 date bug

ninja 1.9.0+ exposes a bug in python shutil.copy2 where a copied file's
date is the src's date but rounded. This can cause ninja to think the
copied file is slightly older than the src and trigger unnecessary work
when building.

https://github.com/ninja-build/ninja/issues/1554

Change-Id: I53247453f38a1c98c5d832bdda0f68effbe786c2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/320261
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

gn/cp.py

@@ -21,3 +21,5 @@ if os.path.isdir(src):
   shutil.copytree(src, dst)
 else:
   shutil.copy2(src, dst)
+  #work around https://github.com/ninja-build/ninja/issues/1554
+  os.utime(dst, None)