feat(dotfiles): add session env registry

configs/nixos/modules/shell/rust.nix

@@ -23,9 +23,9 @@ in
     lib.mkEnableOption "Rust tools for the primary user's shell environment";
 
   config = lib.mkIf config.modules.shell.rust.enable {
-    home-manager.users.${user} = {
-      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+    registry.env.${config.networking.hostName}.RUST_TOOLCHAIN_ROOT = "${toolchain}";
 
+    home-manager.users.${user} = {
       home.packages = [
         toolchain
         pkgs.rust-analyzer