Update copilot-setup-steps.yml to use global.json for .NET SDK version#888

stephentoub merged 2 commits intomainmodelcontextprotocol/csharp-sdk:mainfrom

copilot/update-dotnet-sdk-versionmodelcontextprotocol/csharp-sdk:copilot/update-dotnet-sdk-versionCopy head branch name to clipboard