Add "nixos" to operating system type validator

[shayne]

Describe the bug
NixOS proxmox-lxc containers have OS Type of "nixos".

To Reproduce
Steps to reproduce a CT with the "nixos" OS Type:

1. Generate CT template with nix run github:nix-community/nixos-generators -- --format proxmox-lxc
2. Upload to Proxmox
3. Create CT from the template

To reproduce in Terraform:

1. Create a new "proxmox_virtual_environment_container" resource

2. Specify the "operating_system" to be the template

   operating_system {
      type             = "nixos"
      template_file_id = "local:vztmpl/nixos-system-x86_64-linux.tar.xz"
    }
   

Expected behavior
Validation doesn't fail.

Screenshots

Additional context
Add any other context about the problem here.

• Provider version (ideally it should be the latest version): v0.22.0
• Terraform version: v1.4.6
• OS (where you run Terraform from)): NixOS
• Debug logs (TF_LOG=DEBUG terraform apply):