[CI] Factorize the Apple SDK download
Summary:
The download process is duplicated between the depends build and the
gitian build. This diff makes it a common script so there is a single
place to maintain when the SDK is updated.
Test Plan:
./contrib/teamcity/build-configurations.py build-osx ./contrib/teamcity/build-configurations.py gitian-osx
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7769