update-dependencies cannot update with an internal SDK-only build

[lbussell]

Example of failure: https://dev.azure.com/dnceng/internal/internal%20Team/_build/results?buildId=2838162&view=logs&j=6b443fb8-dee0-558e-a185-67b40a667c8d&t=f9bc539e-e1ec-55fb-e6d6-dc57313d0488&l=7626

Internal build that caused the failure: https://dev.azure.com/dnceng/internal/_build/results?buildId=2838088&view=results

Stack trace:

Unhandled exception: System.Collections.Generic.KeyNotFoundException: The given key 'runtime' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at Dotnet.Docker.FromStagingPipelineCommand.ExecuteAsync(FromStagingPipelineOptions options) in /mnt/vss/_work/1/s/eng/update-dependencies/FromStagingPipelineCommand.cs:line 149
   at Dotnet.Docker.BaseCommand`1.<>c.<<get_Handler>b__4_0>d.MoveNext() in /mnt/vss/_work/1/s/eng/update-dependencies/BaseCommand.cs:line 37

[lbussell]

Fixed by #6823