Add macOS 15 Beta to Scouting Queue

[ilyas1974]

With the release of macOS beta from Apple, we need to add a scouting queue for ARM based systems (similar to osx.amd64.scouting.open)

In addition to macOS 15 beta, we also need to install xcode 16.

Release Note Category

• [x] Feature changes/additions
• [ ] Bug fixes
• [ ] Internal Infrastructure Improvements

  Release Note Description

  Added OSX.15.Arm64.Scouting queue

[AlitzelMendez]

PR: https://dev.azure.com/dnceng/internal/_git/dotnet-helix-machines/pullrequest/40837