Force Python 3 PATH for Appveyor

Author: ivankravets

appveyor.yml

@@ -19,7 +19,7 @@ environment:
 
 install:
 - cmd: git submodule update --init --recursive
-- cmd: SET PATH=%PATH%;C:\Python36\Scripts
+- cmd: SET PATH=C:\Python36\Scripts;%PATH%
 - cmd: pip3 install -U https://github.com/platformio/platformio/archive/develop.zip
 - cmd: platformio platform install file://.