[Lang] Fix a bug on PosixPath (taichi-dev#7860)

When calling `ti gallery,` the `_exec_python_file` function in
`_main.py` simply passes a `pathlib.PosixPath` object to the command.
This may cause problems on Windows platforms. This PR fixes this issue
by forcing using the absolute string path.

python/taichi/_main.py

@@ -215,7 +215,7 @@ def on_mouse_click_callback(example_name):
             content = Syntax.from_path(script, line_numbers=True)
             console = rich.console.Console()
             console.print(content)
-            self._exec_python_file(script)
+            self._exec_python_file(str(script))
 
         index = None
         while not gui.get_event(ti.GUI.ESCAPE, ti.GUI.EXIT):