sdk->installer Repo Consolidation

[MichaelSimons]

The installer repo is being merged into the sdk repo. This impacts source-build as the installer repo was the location of the source-build infrastructure. This infrastructure now resides at https://github.com/dotnet/sdk/tree/main/src/SourceBuild. If you would like make a contribution to the source-build project, please open PRs against the sdk repo going forward.

This is a non-impactful change to distro maintainers and other users who build source-build. You will continue to build from dotnet/dotnet (the VMR) just as before.