main.star: set best_effort_platform on ppc64le

This prevents buildbucket from trying to install Python and Git, which
aren't in CIPD for this platform. See https://crrev.com/i/6261690.

Change-Id: Ide6f08599ff5e31731b89590cf7d6f7bc74327d4
Reviewed-on: https://go-review.googlesource.com/c/build/+/515695
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Heschi Kreinick <[email protected]>
TryBot-Bypass: Heschi Kreinick <[email protected]>

Author: heschi