llvm_tools: fix nightly_revert_checker cherrypick logic

This param should be called `chromeos_tree`. While refactoring that, I
saw that `find_chroot` wasn't a great name for "get the chromeos tree
(or android tree (unused)), interesting SHAs, and email recipients."
Moving that logic into main seems simplest.

BUG=b:333720269
TEST=repo upload

Change-Id: I0b4b0f06a839553f4bad9c8fbcd0b9a209e71f24
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5507516
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Ryan Beltran <ryanbeltran@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
2 files changed