Make darc vmr forwardflow work in VMR insertion PR builds

[premun]

Context

Repositories will have optional build legs which can insert the repository PR branch into the VMR and build it.
In this case, we need tooling similar to the vmr-sync.sh from the dotnet/sdk repo, which does a similar thing nowadays, where it syncs the repo into the VMR (the old way).

More details here:
https://github.com/dotnet/arcade/pull/15354/files#r1893648982

Goal

We need to provide tooling that will enable repos to insert themselves into a VMR during a build:

• Most likely, this will utilize the darc vmr update or darc vmr forwardflow command but we need to make sure it works correctly (even without us supplying a build to it).
• We will need YAML templates and an .sh and .ps1 scripts. These will most likely live in Arcade so that every repo can take advantage of it. Similarly to what we have in dotnet/sdk now these will:
  • Clone the VMR (we have vmr-prepare.yml)
  • Install darc
  • Flow the PR branch into the local VMR
  • Build the VMR (this will be handled by others)