Default to not sign source-build VMR legs

[ViktorHofer]

Right now we explicitly need to set sign: false in the source-build VMR legs. We should make this the default for source-build so that we don't have to set that parameter.

Example: https://github.com/dotnet/sdk/blob/1c88a979b7109d83e5317b0e374c8d13ef2b3b68/eng/pipelines/templates/stages/vmr-build.yml#L103

cc @mmitche @MichaelSimons

[dotnet-issue-labeler]

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[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.