Set GIT_DEPTH to 20

Before if the SHA of the last CI run were ahead of the next CI run by more than 50 commits, the build would fail because Git assumes the full history is present (#33041 (closed)). It does not know the current repository has a shallow clone.

With #39134 (closed), the pre-clone script now seeds the repository with the full history of master and a consistent snapshot. As a result, we should be able to use a shallow clone with a lower depth, which should decrease the load from the server.