fix(nix): set VIRTUAL_ENV for ty to find site-packages

ty needs VIRTUAL_ENV to locate the Python environment's site-packages.
Without this, ty looks for .venv which doesn't exist in the Nix environment.

Also update justfile to call ty directly since it's available in PATH
within the Nix shell.

Author: ryoppippi

flake.nix

@@ -144,6 +144,8 @@
                 UV_NO_SYNC = "1";
                 UV_PYTHON = "${python}/bin/python";
                 UV_PYTHON_DOWNLOADS = "never";
+                # Set VIRTUAL_ENV for tools like ty that look for site-packages
+                VIRTUAL_ENV = "${virtualenv}";
               };
 
               shellHook = ''