Decouple AUR patch recipe from the rest of the automated commits pipeline

Summary:
This is the first step to decoupling the commit generation logic
from the rest of the automation pipeline. Relevant feedback: https://reviews.bitcoinabc.org/D7479#177825
Once complete, the land bot logic that applies a revision will be no different
from any other commit generator.

Depends on D5808

Test Plan:
This bails early and does not run smoke tests:

COMMIT_TYPE=dummy ./automated-commits.sh -p HEAD ./patch-recipes/update-aur-version.sh bitcoin-abc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7490