Verify that stable preview builds work as expected.

[mmitche]

Stable preview builds of the VMR should succeed and generate artifacts as expected.

[mmitche]

https://dev.azure.com/dnceng/internal/_build/results?buildId=2704028&view=results

[mmitche]

Setting StabilizePackageVersion doesn't seem like a good strategy for previews. It was used as a shorthand for repos to entirely chop off the pre-release label, rather than just produce a stable preview. Exploring just setting DotNetFinalVersionKind to prerelease.

But one thing important to note is I'm not sure that it will correctly note that these are stable versions and put them in isolated feeds. It's likely that some of that logic has degraded