[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