Omit benches that time out on Pixel3a

I failed to notice the failing tryjob on
https://skia-review.googlesource.com/c/skia/+/240557

Bug: skia:9413
Change-Id: Iffb83d7210e08107eadee8ffd47158cce3078a1d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240682
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed