[backport#15040] Add workaround for QProgressDialog bug on macOS

Summary:
See: QTBUG-65750, QTBUG-70357.

Backport of Core PR15040

This apparently reapplies a nit fixed in D2144. I've chosen to stay close to the Core version for less merge headaches.

Test Plan: unfortunately I do not have a macOS in hand to test this

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6957