Improve Performance-setup perf repo download selection

Per https://github.com/dotnet/runtime/pull/77699/files#r1012419205. We currently always get the main repo of the perf repository during performance-setup(.sh/.ps1), this causes branches (i.e. release/6.0 and release/7.0) to need to be updated to work. It would be beneficial if we did not have to do this each major release, taking us one step further in automating the system.