fix windows scripts

scripts/build.cmd

@@ -8,10 +8,28 @@ cd /d "%~dp0\.."
 
 set VENV_DIR=.venv-platformio
 
+REM Prefer the "py" launcher (ships with official Python installer) which
+REM reliably picks Python 3, even if "python" on PATH points to an old
+REM Python 2.x install.
+where py >nul 2>nul
+if %errorlevel%==0 (
+    set PYTHON_CMD=py -3
+) else (
+    set PYTHON_CMD=python
+)
+
 if not exist "%VENV_DIR%" (
-    echo Creating local virtualenv in %VENV_DIR% ...
-    python -m venv "%VENV_DIR%"
-    "%VENV_DIR%\Scripts\pip.exe" install --upgrade pip
+    echo Creating local virtualenv in %VENV_DIR% using: %PYTHON_CMD%
+    %PYTHON_CMD% -m venv "%VENV_DIR%"
+    if not exist "%VENV_DIR%\Scripts\python.exe" (
+        echo.
+        echo ERROR: Could not create a Python 3 virtualenv.
+        echo Make sure Python 3 is installed from https://www.python.org/downloads/
+        echo and that "py -3 --version" or "python --version" reports Python 3.x
+        echo ^(your "python" currently points to: %PYTHON_CMD%^)
+        exit /b 1
+    )
+    "%VENV_DIR%\Scripts\python.exe" -m pip install --upgrade pip
     "%VENV_DIR%\Scripts\pip.exe" install platformio
 )