[infra/config] Bump OS X SDK on mac builders to version 10.1 Bug: 973721 Change-Id: Iaead721362a0d19912f0d365654c613ff861c842 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1742543 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>