Here’s how you can accomplish a global gitignore using NixOS (copied from a comment I made down the thread):

 { lib, pkgs, ... }:

  programs.git = {
    enable = true;

    ignores = [
      "*.bloop"
      "*.bsp"
      "*.metals"
      "*.metals.sbt"
      "*metals.sbt"
      "*.direnv"
      "*.envrc"
      "*hie.yaml" 
      "*.mill-version"
      "*.jvmopts"
      "build/" 
    ];
}