[cipd] Always bootstrap mac-amd64, even on arm64.

There's no mac-arm64 yet, since Go doesn't support this. Presumably
mac-amd64 should work fine on ARM Mac too, see the bug.

Note that this will transitively affect all binaries fetched by
CIPD, they all will be mac-amd64.

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

Change-Id: Ie3c5b5776c3d2f83b5dfa411016862dbbc53a9e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2282940
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>

cipd

@@ -30,6 +30,7 @@ case "${UNAME}" in
     ;;
   darwin)
     OS=mac
+    ARCH=amd64  # use amd64 binaries even on arm64, see crbug.com/1102967
     ;;
   *)
     >&2 echo "CIPD not supported on ${UNAME}"