LUCI bucket can take a few different formats, and it's not clear which
one `git cl try` wants. So this clarifies it a bit.

Bug: None
Change-Id: I4816cf17e3af376d447395021fbc3544616c5451
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3624550
Auto-Submit: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>