Update dotnet/versions LKG_Packages file automatically

[dagood]

This file is like Latest_Packages in that old entries aren't cleared (#1118), but for LKG_Packages, updates should only be performed if the current build produced a stable version. /cc @ericstj @chcosta

See dotnet/versions#161 (the LKG file being out of date since it's a manual step)

[ViktorHofer]

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Going forward, the .NET team is using https://github.com/dotnet/arcade to develop the code and issues formerly in this repository. Feel free to reopen/move this issue if it still applies to servicing branches in this repository, or source code which now resides in the Arcade repository.