Right now, our Copilot setup steps try to install a good version of the .NET SDK with this call to setup-dotnet:

However, our global.json is nonstandard: we don't pin an SDK in "the usual way". We use an Arcade-specific tools.dotnet element that setup-dotnet doesn't know about.

Update .github/workflows/copilot-setup-steps.yml to

1. extract the value of tools.dotnet to get an SDK version to install
2. pass that to setup-dotnet

I think that may involve a script using jq and setting a GitHub Actions variable in step 1, then using that variable in step 2, but I'd love to hear about better options.