Allow the cdac build tool to roll forward to whatever SDK is available (prerelease or otherwise)

[jkoritzinsky]

Fixes failures in source-build in dotnet/sdk#43015

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[MichalStrehovsky]

This fixed the cdac-build tool, but dotnet/sdk#43015 is now failing with the same thing on crossgen:

      /vmr/src/runtime/dotnet.sh /vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/x64/crossgen2/crossgen2.dll -o:/vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/System.Private.CoreLib.dll -r:/vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/IL/*.dll --targetarch:x64 --targetos:linux -m:/vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/StandardOptimizationData.mibc --embed-pgo-data -O /vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/IL/System.Private.CoreLib.dll --perfmap-format-version:1 --perfmap --perfmap-path:/vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/
      You must install or update .NET to run this application.
      
      App: /vmr/src/runtime/artifacts/bin/coreclr/linux.x64.Release/x64/crossgen2/crossgen2.dll
      Architecture: x64
      Framework: 'Microsoft.NETCore.App', version '9.0.0-preview.7.24405.7' (x64)
      .NET location: /vmr/.dotnet/
      
      The following frameworks were found:
        10.0.0-alpha.1.24504.10 at [/vmr/.dotnet/shared/Microsoft.NETCore.App]

Of course I could go add --rollforward to this crossegen invocation, but I already see we then need to merge IBC files that will likely also run into this problem.

Is there some other mechanism we could use? For example an environment variable we could just set in build.cmd/build.sh? If there is, any drawbacks with that?

[jkoritzinsky]

If we update dotnet-runtime to use a .NET 10 alpha SDK and bump the "Tool current" TFM, that would also work.

I don't think we want to set the environment variable to roll forward automatically as it would allow us to forget to update the target TFM and end up running in an unexpected scenario.