To switch a depot_tools installation to use mac-arm64 binaries,
run:

  $ cd depot_tools
  $ echo "mac-arm64" > .cipd_client_platform
  $ ./cipd help
  $ <observe "infra/tools/cipd/mac-arm64" in the version string>

Deleting .cipd_client_platform file restores the original logic
(which is currently to use mac-amd64 binaries even on arm64 host).

R=iannucci@chromium.org, tandrii@chromium.org
CC=thakis@chromium.org
BUG=1102967

Change-Id: I0248fd913eb3d71f2a228f74a601dc4b12fc8824
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2699692
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>