build: check if python is a executable program

After Windows 10 version 1903, placeholder files named "python.exe"
and "python3.exe" appear, not executable programs, and open the
MS Store when running without parameters.

Fixes: #36694
Refs: https://devblogs.microsoft.com/python/python-in-the-windows-10-may-2019-update

PR-URL: #36696
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/msvs/find_python.cmd

@@ -46,6 +46,8 @@ exit /b 1
 
 :found-python
 echo Python found in %p%\python.exe
+call :check-python %p%\python.exe
+if errorlevel 1 goto :no-python
 endlocal ^
   & set "pt=%p%" ^
   & set "need_path_ext=%need_path%"
@@ -54,6 +56,15 @@ set "pt="
 set "need_path_ext="
 exit /b 0
 
+:check-python
+%~1 -V
+:: 9009 means error file not found
+if %errorlevel% equ 9009 (
+  echo Not an executable Python program
+  exit /b 1
+)
+exit /b 0
+
 :no-python
 echo Could not find Python.
 exit /b 1