nixos: One-click install via disko-install

[srid]

WIP

https://nixos.asia/en/nixos-install-oneclick

sudo nix --extra-experimental-features 'flakes nix-command' \
  run github:nix-community/disko#disko-install -- \
  --flake github:juspay/nixos-unified-template/disko#example \
  --write-efi-boot-entries \
  --disk main /dev/sda \
  --system-config '{"nixpkgs": {"hostPlatform": "aarch64-linux"}}'

(That last line is only necessary if on aarch64 linux VM)

[srid]

Hitting https://github.com/nix-community/disko/issues/947#issuecomment-2685647104