Allow the SDK to restore SourceLink and find its MSBuild props and targets files. These are used to fetch Git information.

Use Arcade-generated '_version.c'. The new '_version.c' is identical to the old 'version.cpp'. The host build.sh generated an empty 'version.cpp' when it doesn't already exist at the specified path, which caused the "No version information" message to be included.

Commit migrated from https://github.com/dotnet/core-setup/commit/bac3b0dc8ec9e0d4d2441524ed2bca24f8bab748