dina-font: fix the installPhase

The existing expression worked fine as a user but seems broken when built by hydra and when in systemEnvironment:
https://hydra.nixos.org/build/17703539/log

I'm not sure why, but hopefully defining the function in the installPhase will fix it?

Is there an easy way to include my ~/.nix-defexpr/ (which has my nixpkgs repo) in the packages searched for systemEnvironment.pkgs?