pw_toolchain: Fix copy rules on Mac
pw_toolchain: Fix copy rules on Mac

Copy rules are always dirty on Mac because cp -af doesn't preserve the
timestamps with full fidelity.

Change-Id: Ib7b7e346ca46bebe5e38832ae491fafeedfd0035
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/12665
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed