Implement a strategy to publish productVersion.txt produced by the vertical build

[MilenaHristova]

Related issues: #4199 and #4157

One vertical creates multiple productVersion.txt files - Sdk\[VERSION]\productVersion.txt, WindowsDesktop\[VERSION]\productVersion.txt, aspnetcore\Runtime\[VERSION]\productVersion.txt. Those files are used by dotnet-install scripts.

In the final build pass we merge the assets from all verticals and copy them to a flat layout in BlobArtifacts folder. We can't have multiple files with the same filename. We should figure out a way to select the files to publish and publish them in the locations expected by dotnet-install scripts or alternatively, modify the scripts.

There is also Sdk\[VERSION]\sdk-productVersion.txt, WindowsDesktop\[VERSION]\windowsdesktop-productVersion.txt, aspnetcore\Runtime\[VERSION]\aspnetcore-productVersion.txt that could potentially be used instead of productVersion.txt