Use lkgm file for Lacros public builders

Use lkgm version from //chromeos/CHROMEOS_LKGM instead of
the buildbot config.

Change-Id: Iece7ed466cad621eee91e10e6293c8c409f3f724
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384923


Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Kuan Huang <kuanhuang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278672}