treewide: move FLAKE env variable to core/nix

hosts/solaire/default.nix

@@ -22,6 +22,4 @@ _: {
       nvidia.enable = true;
     };
   };
-
-  environment.variables.FLAKE = "/home/nezia/.dotfiles";
 }

hosts/vamos/default.nix

@@ -25,6 +25,4 @@ _: {
       hyprland.enable = true;
     };
   };
-
-  environment.variables.FLAKE = "/home/nezia/.dotfiles";
 }

modules/core/nix.nix

@@ -1,4 +1,12 @@
-{pkgs, ...}: {
+{
+  lib,
+  pkgs,
+  config,
+  ...
+}: let
+  inherit (lib) mkIf;
+  inherit (config.local.systemVars) username;
+in {
   nix = {
     package = pkgs.lix;
     settings = {
@@ -32,4 +40,10 @@
       segger-jlink.acceptLicense = true;
     };
   };
+
+  hjem.users.${username} = {
+    files.".config/environment.d/${config.local.homeVars.userEnvFile}.conf".text = ''
+      FLAKE="${config.hjem.users.${username}.directory}/.dotfiles"
+    '';
+  };
 }