SBE failing due to use of preview version of .NET

[ellahathaway]

This PR build targeting preview7 failed with the following error while building source build externals:

/vmr/.dotnet/sdk/9.0.100-preview.7.24357.2/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.RuntimeIdentifierInference.targets(326,5): message NETSDK1057: You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy [/vmr/src/source-build-externals/artifacts/sb/src/src/spectre-console/src/Spectre.Console/Spectre.Console.csproj::TargetFramework=net9.0]
      /vmr/.dotnet/sdk/9.0.100-preview.7.24357.2/Roslyn/Microsoft.CSharp.Core.targets(89,5): error MSB6006: "csc.dll" exited with code 139. [/vmr/src/source-build-externals/artifacts/sb/src/src/spectre-console/src/Spectre.Console/Spectre.Console.csproj::TargetFramework=net9.0]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ellahathaway]

https://github.com/dotnet/dotnet/blob/76d842118ea686c6d31a842a392d53a4680b2f91/Directory.Build.props#L125 is set, investigating why it didn't propagate/maybe got overridden.