All packages except mac-amd64 ones are built with go1.16. mac-amd64
are built with go1.15, to preserve OSX 10.10/10.11 compatibility.

Note that we still use mac-amd64 even on arm64 OSX, since
the bootstrap script still installs mac-amd64 version of CIPD.

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

Change-Id: I8fdb27397fbbf238fe39e81305d4642a0500ab33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2699690
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>