fix(fish): export Rust toolchain root

configs/nixos/modules/shell/rust.nix

@@ -23,13 +23,13 @@ in
   options.modules.shell.rust.enable = lib.mkEnableOption "Rust tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
+    registry.env-var.${config.networking.hostName}.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+
     home-manager.users.${user} = {
       home.packages = [
         toolchain
         pkgs.rust-analyzer
       ];
-
-      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
     };
   };
 }