The new version is built with an older Xcode version for mac-amd64,
using the 10.15 SDK, which gives us better Big Sur compatibility for
some of the very old Python libraries we have in use. In particular,
it will see the OS version as 10.16 rather than 11.0, and recognize
the 10.x-built vpython wheels as compatible.

Bug: 1175200
Change-Id: I23be64b11c8a2fb31910ce5e13b98bd5b38eeb7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2876311
Auto-Submit: Brian Ryner <bryner@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>