nixos/railcar: remove use of the deprecated string type
nixos/railcar: remove use of the deprecated string type

This fixes the warning being emitted by nixos-rebuild switch:

building Nix...
building the system configuration...
trace: warning: types.string is deprecated because it quietly concatenates strings

It started emitting a warning in #66346.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs