Fix the CI gitian build script when the OSX SDK is should be downloaded

Summary: The syntax testing the hash of the archive is incorrect.

Test Plan: Run the OSX Gitian build (needs D5530).

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5532