[ArPow] Re-enable sourcelink

[dagood]

dotnet/sourcelink#629 is an issue for the current source-build plan, worked around by disabling sourcelink in dotnet/source-build-reference-packages#133.

When fixed and uptaken by Arcade, we can enable sourcelink in 5.0.

(Filing this a little early, to make sure it ends up tracked.)

[omajid]

Sourcelink is enabled in .NET 6 and 7, which is why we discovered issues like #2903

IMO, we should close this issue now.