Enable offline build in PR validation for .NET 8.0

[MichaelSimons]

Offline tarball build CI is currently disabled for .NET 8.0 because there are prebuilts. This needs to be enabled to ensure source-build can be released for each of the .NET 8.0 previews.

Current Blocking Issues/PRs:

• Eliminate source-build prebuilts introduced by new test framework infrastructure templating#5519
• ILCompiler projects are causing source-build prebuilts runtime#77601
• Eliminate MicrosoftCodeAnalysis prebuilts in runtime installer#14844
• SuperFileCheck causes runtime.linux-x64.Microsoft.NETCore.Runtime.JIT.Tools prebuilt in source builds runtime#77920
• Update Bundled80Templates to all be 8.0 versions installer#14897

[MichaelSimons]

[Triage] Enabling this is going to cause a log jam of sorts in the dependency flows into installer if prebuilt detection is not enabled at the repo level. We should hold off on enabling this until prebuilt detection is enabled for at least the primary repos (e.g. runtime, aspnet, roslyn, sdk, etc.)