[CI] Run coderabbit on diffs

Summary:
Now that the build bot is deployed, we can remove the cat: the message is saved to an artifact file and printed as a comment. It can run on all diffs automatically, so the runOnDiff flag is set.
The base commit change is from testing: on staging master is not necessarily the proper base so always diff on the last commit instead, which works in all cases.

Test Plan: See the result on this diff (need to summon the bot, as it won't run automatically until this is landed).

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D20355