Now that Goma is part of depot_tools (and any stale standalone
checkouts in ~/goma are dysfunctional), update gm.py's detection
logic.
Note: this only affects new args.gn files created by gm.py. On
machines where the build is already set up, this has no effect.

Fixed: v8:11160
No-Try: true
Change-Id: I19f475a51d4345d803d49d3ad2720a0f4f6f84bd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2735637Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73194}