Only check out the GIT repo twice and reuse for the entire pipeline (dotnet/coreclr...

Only check out the GIT repo twice and reuse for the entire pipeline (dotnet/coreclr#26480)

The purpose of this change is to increase the throughput and
reliability of CoreCLR CI pipelines by only performing one GIT
checkout for Windows and Unix, zip them up and reuse for all
subsequent phases / jobs in the pipeline.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/17a53cb063b5e63fc1cc832ec36cf8a520111124