[infra/config] Add cache for XCode on the mac builders Change-Id: Ib4918c58aa63775c63b8d6852cf99264e629c040 Bug: None Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1745721 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>