Nix VM with Debian + GUI

Username: `debian`
Password: `nixos`

0. Clean the VM:
`nh clean all`
`rm ~/.local/share/debian-vm/debian-persistent.qcow2`

1. Build the VM:
```bash
nix-build vm.nix -A system.build.customVM
```
2. Run the VM:
```bash
./result/bin/run-debian-vm
```