Fix OSX SDK caching in Gitian builds

Summary:
D6976 makes a change so that the location of OSX_SDK gets
cleaned up before every build, effectively eliminating the cache.

This patch moves OSX_SDK to an explicit a cache directory instead.

The cached file needs to be copied since the build container fails to
interpret a symlink.

Test Plan:
OS_NAME contrib/teamcity/gitian.sh
Run the above twice. The second time should not download OSX_SDK

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6997