chore: Add base configuration.

system/nix.nix

@@ -0,0 +1,52 @@
+{
+  lib,
+  config,
+  outputs,
+  inputs,
+  ...
+}: {
+  nixpkgs.config = {
+    allowUnfree = true;
+    allowUnfreePredicate = _: true;
+    cudaSupport = true;
+  };
+  nixpkgs.overlays = [
+    outputs.overlays.additions
+    outputs.overlays.modifications
+    outputs.overlays.unstable-packages
+  ];
+  nix = let
+    flakeInputs = lib.filterAttrs (_: lib.isType "flake") inputs;
+  in {
+    settings = {
+      # Enable flakes and new 'nix' command
+      experimental-features = "nix-command flakes";
+      # Opinionated: disable global registry
+      flake-registry = "";
+      # Workaround for https://github.com/NixOS/nix/issues/9574
+      nix-path = config.nix.nixPath;
+      # allowUnfree = true;
+      auto-optimise-store = true;
+      builders-use-substitutes = true;
+      keep-derivations = true;
+      keep-outputs = true;
+      trusted-users = ["root" "@wheel"];
+
+      substituters = [
+        "https://cache.nixos.org"
+        "https://nix-community.cachix.org"
+      ];
+
+      trusted-public-keys = [
+        "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      ];
+    };
+    # Opinionated: disable channels
+    channel.enable = false;
+
+    # Opinionated: make flake registry and nix path match flake inputs
+    registry = lib.mapAttrs (_: flake: {inherit flake;}) flakeInputs;
+    nixPath = lib.mapAttrsToList (n: _: "${n}=flake:${n}") flakeInputs;
+  };
+}