Enable sourcelink in source-build to produce .version file in shared fx

[dagood]

• For Investigate 6.0 SDK missing file: shared/Microsoft.NETCore.App/6.0.0/.version source-build#2569
• For [ArPow] Re-enable sourcelink source-build#1693

In 6.0.100, the source-build team noticed the .version file in the source-built shared framework was missing. The file should contain the commit hash and version number. Enabling sourcelink is the clear way to get that infrastructure back online.

---

Enabling sourcelink regresses a dev scenario for Arcade-powered source-build local builds: due to a sourcelink bug, you must use origin as a remote name:

• When the "origin" remote points to a local directory that itself doesn't have an "origin" remote, SourceLink emits warning/error even though it has a fallback sourcelink#629

This probably doesn't affect many people and it's not impossible to adapt to it, so it seems worth going ahead with it anyway.

[dotnet-issue-labeler]

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