fontconfig: unable to disable font substitions.

[pbogdan]

Issue description

Documentation of fonts.fontconfig.ultimate.substitutions suggests disabling substitutions should be possible by setting the option's value to "none". This fails with the following error:

building Nix...                                      
building the system configuration...
error: The option value `fonts.fontconfig.ultimate.substitutions' in `/etc/nixos/configuration.nix' is not a null or one of "free", "combi", "ms".
(use ‘--show-trace’ to show detailed location information)

Trying to set the option's value to null produces:

building Nix...
building the system configuration...
error: cannot coerce null to a string, at /home/pbogdan/nixpkgs/nixos/modules/config/fonts/fontconfig-ultimate.nix:81:54

Steps to reproduce

Add one of the following to /etc/nixos/configuration.nix:

• fonts.fontconfig.ultimate.substitutions = "none";
• fonts.fontconfig.ultimate.substitutions = null;

Technical details

https://github.com/NixOS/nixpkgs/blob/release-17.03/nixos/modules/config/fonts/fontconfig-ultimate.nix#L80
https://github.com/NixOS/nixpkgs/blob/release-17.03/nixos/modules/config/fonts/fontconfig-ultimate.nix#L155

There appears to be a mismatch in the type definition of the option and how it is being used within the module.

• System: NixOS 17.03 @ 2e983f1 + plus few custom commits not related to fontconfig
• Nix version: 1.11.11
• Nixpkgs version: 2e983f1
• Sandboxing enabled: no