running Install-Product only in windows box

has error on linux cause linux doesn't have powershell of course

Author: Almenon

appveyor.yml

@@ -23,7 +23,7 @@ environment:
       PYTHON: "C:\\Python36-x64"
 
 install:
-  - ps: Install-Product node $env:nodejs_version
+  - cmd: powershell Install-Product node $env:nodejs_version
   - sh: nvm install $nodejs_version
   - npm install