common/nix: chanage nixpkgs flake to github ref

This makes it so flake.lock files produced by `nix flake lock` don't
reference my personal Nix store, but upstream Nixpkgs

system/common/default.nix

@@ -5,5 +5,6 @@
     ./me.nix
     ./remote.nix
     ./wm.nix
+    ./nix.nix
   ];
 }

system/common/nix.nix

@@ -0,0 +1,65 @@
+{ pkgs, lib, config, options, inputs, ... }:
+let
+  cfg = config.nixfiles.common.nix;
+in
+{
+  options.nixfiles.common.nix = {
+    enable = lib.mkEnableOption "common Nix configuration";
+    registerNixpkgs = lib.mkOption {
+      type = lib.types.bool;
+      default = cfg.enable;
+      example = "true";
+      description = "Whether to register the Nixpkgs revision used by Nixfiles to the system's flake registry and make it tye system's <nixpkgs> channel";
+    };
+    /* # TODO
+    register = lib.mkOption {
+      type = lib.types.bool;
+      default = cfg.enable;
+      example = "true";
+      description = "Whether to register Nixfiles to the system's flake registry";
+    };
+    */
+  };
+
+  config = lib.mkMerge [
+    ( lib.mkIf cfg.registerNixpkgs {
+
+      # this makes modern nix tools use the system's version of nixpkgs
+      nix.registry = {
+        nixpkgs = {
+          exact = true;
+          from = {
+            id = "nixpkgs";
+            type = "indirect";
+          };
+
+          # used instead of `flake` option so produced flake.lock files are
+          # portable
+          to = {
+            type = "github";
+            owner = "NixOS";
+            repo = "nixpkgs";
+            rev = "${inputs.nixpkgs.rev}";
+          };
+        };
+      };
+
+      # this makes comma and legacy nix utils use the flake nixpkgs for ABI
+      # compatibility becasue once `, vkcube` couldn't find the correct opengl
+      # driver or something (also it reduces the download size of temporary shell
+      # closures)
+      nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ] ++ options.nix.nixPath.default;
+    })
+    ( lib.mkIf cfg.enable {
+
+      # direnv is a tool to automatically load shell environments upon entering
+      # a directory. nix-direnv has an extensionn to keep nix shells in the
+      # system's gcroots so shells can be used after a gc without rebuilding.
+      programs.direnv.enable = lib.mkDefault true;
+
+      # fallback to building locally if binary cache fails (home-manager should be
+      # able to handle simple rebuilds offline)
+      nix.settings.fallback = lib.mkDefault true;
+    })
+  ];
+}

system/profile/base.nix

@@ -11,24 +11,24 @@ in
   config = lib.mkMerge [
     (lib.mkIf cfg.enable {
 
-      # Enable my account
-      nixfiles.common.me.enable = lib.mkDefault true;
+      nixfiles.common = {
+        # Enable my account
+        me.enable = lib.mkDefault true;
+        # Enable system Nix configuration
+        nix.enable = lib.mkDefault true;
+      };
 
       # locale settings
       i18n = {
-        defaultLocale = "en_US.UTF-8";
+        defaultLocale = lib.mkDefault "en_US.UTF-8";
         extraLocaleSettings = {
-          LC_ALL = "en_US.UTF-8";
+          LC_ALL = lib.mkDefault "en_US.UTF-8";
         };
       };
 
       # Enable flakes
       nix.settings.experimental-features = ["nix-command" "flakes" ];
 
-      # fallback to building locally if binary cache fails (home-manager should be
-      # able to handle simple rebuilds offline)
-      nix.settings.fallback = true;
-
       # Allow unfree packages
       nixpkgs.config.allowUnfree = true;
 
@@ -81,16 +81,10 @@ in
         sops
       ];
 
-      programs.neovim.defaultEditor = true;
+      programs.neovim.defaultEditor = lib.mkDefault true;
 
-      # this makes comma and legacy nix utils use the flake nixpkgs for ABI
-      # compatibility becasue once `, vkcube` couldn't find the correct opengl
-      # driver or something (also it reduces the download size of temporary shell
-      # closures)
-      nix.nixPath = [ "nixpkgs=${inputs.nixpkgs}" ] ++ options.nix.nixPath.default;
-
-      programs.ssh.enableAskPassword = false;
-      programs.fuse.userAllowOther = true;
+      programs.ssh.enableAskPassword = lib.mkDefault false;
+      programs.fuse.userAllowOther = lib.mkDefault true;
 
       programs.gnupg.agent = {
         enable = lib.mkDefault true;