gst-env: import exit from sys

On windows the Cmd Prompt for VS 2019 complains
that exit is not defined.

  File "C:/data/gstreamer/gst-env.py", line 622, in <module>
    exit(subprocess.call(args, close_fds=False, env=env))

NameError: name 'exit' is not defined
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3324>

gst-env.py

@@ -14,6 +14,7 @@ import pathlib
 import signal
 from functools import lru_cache
 from pathlib import PurePath, Path
+from sys import exit
 
 from typing import Any