Update appveyor.yml

Author: josesimoes

appveyor.yml

@@ -80,7 +80,7 @@ before_deploy:
 
 # requires APPVEYOR_DISCORD_WEBHOOK_URL enviroment variable set with Discord webhook URL
 on_failure:
-  - ps: $env:APPVEYOR_BUILD_FOLDER\appveyor-discord.ps1 failure $env:APPVEYOR_DISCORD_WEBHOOK_URL
+  - ps: '& "$env:APPVEYOR_BUILD_FOLDER\appveyor-discord.ps1 failure $env:APPVEYOR_DISCORD_WEBHOOK_URL"'
 
 cache:
   - source\packages -> **source\packages.config