Repo source-build should copy sources instead of git clone

[ViktorHofer]

https://github.com/dotnet/arcade/blob/b372abae012cab2dbf011bd39c395e940e35da86/src/Microsoft.DotNet.Arcade.Sdk/tools/SourceBuild/SourceBuildArcadeBuild.targets#L98

CopySrcInsteadOfClone should probably default to true?

cc @NikolaMilosavljevic

[MichaelSimons]

Why is copy preferred over clone?

[ViktorHofer]

When I talked with @NikolaMilosavljevic offline he mentioned that copying should be preferred over git clone. @NikolaMilosavljevic can you please provide some arguments on that?

For me the obvious ones are avoiding the unnecessary networking calls and the extra work to resolve the git deltas. We already checked the repository out from git so why do it again when the sources are already on disk?

[NikolaMilosavljevic]

I don't have any "hard" data myself. I've heard few times that copy would solve some issues, but I don't have a preference here.

[MichaelSimons]

@ViktorHofer points out in dotnet/runtime#98476 how some sourcelink changes require copy instead of clone. This raises a question to me if the clone option should be retained or if we should only support the copy option?

[MichaelSimons]

Per in-person discussion we would like to validate to ensure there are no scenarios that require the clone - e.g. repos with submodules. If no repos require clone, we feel it is best to remove it and only have the copy path.