[infra] Use Mac10.15.7 and xcode 12.2 for CommandBuffer builds.

Bug: skia:11082
Change-Id: I5ba2f9ae85ce7e008c2fc656be605c022466720d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343938
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Weston Tracey <westont@google.com>
6 files changed