Add orichard-nixbuild to the refrepo

Renaming the users-vm.yaml file to user-machines.yaml has no impact (indifferently a separate file could be created if preferred).

Just need to update https://www.grid5000.fr/w/TechTeam:Persistent_Virtual_Machine_admin.

Tested ok with additional information such as:

• contact email (really useful ? or just rely on the username and user's API?)
• location
• model
• arrival date
• ...