Support `darc vmr reset` outside of vmr directory similar to `darc vmr forwardflow`

[ViktorHofer]

See discussion in dotnet/dotnet#4897

It would be good if darc vmr reset could be called from the repository that should be reset in the vmr. This is already possible with darc vmr forwardflow. We should align the two commands.

This will allow us simplifying the repo name calculation in the vmr-sync scripts.