[qa] Add shrinkdebugfile=0 to regtest bitcoin.conf

Summary:

This helps avoid accidentally truncating the debug.log while manuallyd debugging.

This is a backport of Core PR17330

Test Plan: ninja && ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8233