Automatically create new VMR-lite branches in source-build-release pipeline

[lbussell]

This should go in the new source-build-pre-release pipeline.

Use the dotnet source tarball from azure blob storage that will be uploaded by #2710 (it could also work if it is uploaded manually). I already have a script I use to run the update manually here: https://gist.github.com/lbussell/fb72c5b32fef03c3f9de2e5b5e466a3c

This may require creating a bot for our internal security partners repo.

The steps required to completely automate this are:

1. Download dotnet source tarball from azure storage
2. Run a script similar to the one above to create a new update branch
3. Push the branch to dotnet-security-partners