cipd: Add a note about .cipd_client_platform

No behavior change.

Bug: 1102967
Change-Id: Ic5aca441ee62f9c61a06cf2a0d510a8b72733688
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2778081
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>

cipd

@@ -30,7 +30,11 @@ case "${UNAME}" in
     ;;
   darwin)
     OS=mac
-    ARCH=amd64  # use amd64 binaries even on arm64, see crbug.com/1102967
+    # use amd64 binaries even on arm64 for now, see crbug.com/1102967
+    # Once we have arm binaries of everything, this will change to "arm64"
+    # Run `echo "mac-arm64" > .cipd_client_platform` to force arm64 binaries
+    # before everything is ready, for testing.
+    ARCH=amd64
     ;;
   *)
     >&2 echo "CIPD not supported on ${UNAME}"