[infra] Move iPhone11 jobs to iOS-13.6.

13.6 requires xcode >11.4.1, which requires MacOS > 10.15.2.
Use new Mac10.15.5 build jobs for iPhone11 tasks, and on those
jobs use chromium's mac_toolchain to compile with xcode 11.4.1.

Change-Id: I988197d7ba4c97b9facfe8b93f87878eba806de8
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300198
Commit-Queue: Weston Tracey <westont@google.com>
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
6 files changed