Enable EmbedUntrackedSources throughout Roslyn

[RikkiGibson]

In order to rebuild Roslyn, we need to be able to access sources generated at build time, such as AssemblyInfo.cs, as well as sources produced by source generators like CSharpSyntaxGenerator.

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• Once we update to Arcade 6 we should remove our workaround that sets EmbedUntrackedSources to false: Embed untracked sources in all non-WPF projects #51353 (comment)

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This is a temporary change correct? The idea being that when we are done with this effort that we will be adding this to Directory.Build.props? If so are we tracking that in an issue, comment, etc ...?

Originally posted by @jaredpar in #50968 (comment)

[clairernovotny]

Does Roslyn use Arcade? I'd expect EmbedUntrackedSources to be enabled by default by the top-level Arcade SDK.

[jaredpar]

We do use Arcade but we don't use Arcade 5 because it's not LTS. We are still on Arcade 1 (might have been called 3) because it's the only available LTS.

[RikkiGibson]

We are still on Arcade 1 (might have been called 3)

Arcade 1.x is what ships in the .NET 3 Eng channel. https://github.com/dotnet/arcade/blob/release/3.x/eng/Versions.props
Arcade 5.x ships in the .NET 5 channel. https://github.com/dotnet/arcade/blob/release/5.0/eng/Versions.props

I imagine the Arcade maintainers decided to make the version align with the .NET version starting with .NET 5.

[RikkiGibson]

Depends on #51647