Re-enable clean-while-building

[MichaelSimons]

A recent SDK upgrade in dotnet/installer#15372 broke the clean-while-building functionality. The option had an odd side-effect which broke the runtime build which appears that it may be timing/concurrency related (see PR discussion for background). The option was disabled to unblock prebuilt removal for the preview 1 release. This issue is to track investigating the issue and re-enabling the functionality.

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