Fix pygments for nixpkgs-unstable

Looks like some defaults have to be specified manually now for our custom build of pygments. **25.05** ``` nix build --no-link --print-out-paths warning: Git tree '/home/fmzakari/code/github.com/fzakaria/nix-auto-follow' is dirty /nix/store/7f3v9dbi99ba1c5myy903rq52d2k71m3-nix-auto-follow ``` **unstable** ``` nix build --override-input nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable --no-link --print-out-paths warning: Git tree '/home/fmzakari/code/github.com/fzakaria/nix-auto-follow' is dirty warning: not writing modified lock file of flake 'git+file:///home/fmzakari/code/github.com/fzakaria/nix-auto-follow': • Updated input 'nixpkgs': 'github:NixOS/nixpkgs/88983d4b665fb491861005137ce2b11a9f89f203?narHash=sha256-7orTnNqkGGru8Je6Un6mq1T8YVVU/O5kyW4%2Bf9C1mZQ%3D' (2025-07-08) → 'github:NixOS/nixpkgs/7c688a0875df5a8c28a53fb55ae45e94eae0dddb?narHash=sha256-vtTM4oDke3SeDj%2B1ey6DjmzXdq8ZZSCLWSaApADDvIE%3D' (2025-07-20) /nix/store/3kppajh4g0930yk24rxb9mahlcqxpsgs-nix-auto-follow ``` fixes #29