Lacros: Add lacros-amd64-generic-chrome-skylab in src/

This CL adds the mb config and test specs for a new builder that runs
Lacros tests using the Skylab pipeline.

Bug: 1218582
Change-Id: Ifee5e517d738887e3b5ea8f4c3ba3c3a3669c5b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3121631


Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Xinan Lin <linxinan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915898}