Add build parameter to specify default TargetLatestAspNetCoreRuntimePatch value (source-build)

[dagood]

Helps fix dotnet/source-build#586. (Read the description for why we need to be able to set a default specific to source-build.)

A way I saw to do this is: dagood/cli@4b89017 (using Microsoft.NETCoreSdk.BundledVersions.props to carry a source-build default value for TargetLatestAspNetCoreRuntimePatch). I don't know if it fits what the file's for.

@livarcocc does that look reasonable, or would you suggest something else?

/cc @tmds @natemcmaster

[natemcmaster]

We would also need to make changes in aspnet/websdk (cc @vijayrkn @JunTaoLuo )

See https://github.com/dotnet/cli/issues/9469

[marcpopMSFT]

Create a new bug if we hit issues during the 5.0 pass for source build.