[infra] Switch Mac builders to 10.14.6.

Bug: skia:9194
Change-Id: I33acb878528c461f51989f2a65df7b33ea38f2a7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240277
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed