[infra/bots] Treat extra_config as a list.

Dependency of https://skia-review.googlesource.com/c/14525/

No-Try: true
Change-Id: I7686576aa48865116fe8a593f08d395f901a1d04
Reviewed-on: https://skia-review.googlesource.com/14524
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
3 files changed