Leftover changes after rebasing my runtime changes against CoreCLR (dotnet/coreclr...

Leftover changes after rebasing my runtime changes against CoreCLR (dotnet/coreclr#27751)

1) Viktor moved the empty.csproj script to the repo root, reflect
the change;

2) In xplat-pipeline-job, pick coreClrRepoRoot dynamically based on
pipeline name (thanks Viktor for the suggestion!);

3) Some fixes to call dotnet through the scripts we recently fixed.

4) Remove superfluous echo in dotnet.cmd per Viktor's feedback.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/94b2a0dcd3d00d5d6a7600455c93e467d208b96c