[CI] Fix the OSX cross build

Summary: The SDK download link is down and causes the build to fail. This diff makes the link a variable to be provided by the user, since distributing the SDK is prohibited by Apple.

Test Plan:

../contrib/teamcity/build-configurations.py build-osx
../contrib/teamcity/build-configurations.py gitian-osx

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10194