CoreCLR pipeline optimizations (dotnet/coreclr#27059)

CoreCLR pipeline optimizations (dotnet/coreclr#27059)

Today, R2R compilation of test MSIL using Crossgen happens during
test execution in Helix. This means that the managed artifacts for a
given OS / architecture / config combo are identical for a R2R /
non-R2R job. Additionally, we need very few artifacts for running
the CoreFX tests. In practice it turns out we only need to build the
test host. We can exploit these facts to further reduce the number
of test build jobs in CoreCLR pipelines.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/446c6fe0bfae263bee0c3ace46a5b9a74015e4d1