[style] Code style fixups in GetWarnings()

Summary:
Rename a variable.

This is a partial backport of Core PR17750 [3/3]
https://github.com/bitcoin/bitcoin/pull/17750/commits/7aab8d1024996c7c422bd34a8226df0117b813f7

Depends on D8330

Test Plan: ninja

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8331