Update venv activate.bat to escape custom PROMPT variables on Windows (GH-114885)

Author: GilgameshxZero

Lib/venv/scripts/nt/activate.bat

@@ -15,8 +15,8 @@ if not defined PROMPT set PROMPT=$P$G
 if defined _OLD_VIRTUAL_PROMPT set PROMPT=%_OLD_VIRTUAL_PROMPT%
 if defined _OLD_VIRTUAL_PYTHONHOME set PYTHONHOME=%_OLD_VIRTUAL_PYTHONHOME%
 
-set _OLD_VIRTUAL_PROMPT=%PROMPT%
-set PROMPT=(__VENV_PROMPT__) %PROMPT%
+set "_OLD_VIRTUAL_PROMPT=%PROMPT%"
+set "PROMPT=(__VENV_PROMPT__) %PROMPT%"
 
 if defined PYTHONHOME set _OLD_VIRTUAL_PYTHONHOME=%PYTHONHOME%
 set PYTHONHOME=