Specify cpu for opt guide chromeos tests.

Bug: 329102565
Change-Id: I2fc3cda32bb678070d6abf8f605e41c7c0d1e685
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407253


Commit-Queue: Marco Georgaklis <mgeorgaklis@google.com>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280312}