Increase checkout fetchDepth to 5 (dotnet/coreclr#26774)

Increase checkout fetchDepth to 5 (dotnet/coreclr#26774)

Based on recommended practice and on Jared's experience from the
Roslyn repo I propose increasing the fetchDepth constant to 5 to
prevent occasional checkout failures seen after my change from
last week to only check out the GIT branch once for Windows
and once for Linux per pipeline.

This change is expected to fix the issue

https://github.com/dotnet/coreclr/issues/26733

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/85e7c7497edcf98b720ccd2a6090a0a5cc7fc490