Follow-up on VerifySourceLinkFileExists change in runtime

[crummel]

Runtime commit dotnet/runtime@ff5840b introduced a check for a native sourcelink file that broke installer's uptake of SDK in dotnet/installer#13860. I added a repo.proj parameter VerifySourceLinkFileExists=false to return source-build to the old behavior. Some questions remain about the problem and fix:

• Is this a file that the source-build should produce or has produced in the past?
  • No, source-build has not supported this is in the past.
• Is this a file that the source-build can produce?
  • Possibly. We need to produce the native bits in SourceLink which are currently not part of the build (https://github.com/dotnet/sourcelink/blob/cd036fc6491ac9cacaa9ab2b1a11bb1870ec3c19/README.md#using-source-link-in-c-projects).
• Is there a visible effect on the SDK output?
  • Yes, native files do not have embedded sourcelink metadata.
• Why did the runtime repo source-build leg not fail?
  • ContinuousIntegrationBuild is not set to true in the source-build leg.

If this is the correct fix, it should also be ported to runtime's eng/SourceBuild.props so that the runtime repo leg runs the same way as source-build's runtime build.