After investigating adding a project argument to override the
mapping of libs to packages we realised we could fix the X11
package by updating `modules/configuration-nix.nix`.
We also updated `system-nixpkgs-map.nix` to allow multiple
packages to be mapped for a single library (turned out not
to be necessary for X11).
Co-authored-by: Rodney Lorrimar <rodney.lorrimar@iohk.io>
Co-authored-by: Hamish Mackenzie <Hamish.Mackenzie@iohk.io>