Add .NET CLI configuration to appveyor.yml

This change adds a couple of environment variables to the appveyor.yml to
speed up builds of PowerShell Editor Services by disabling package caching
and telemetry.

Author: daviwil

appveyor.yml

@@ -8,6 +8,10 @@ branches:
   - master
   - develop
 
+environment:
+  DOTNET_SKIP_FIRST_TIME_EXPERIENCE: true  # Don't download unneeded packages
+  DOTNET_CLI_TELEMETRY_OPTOUT: true        # Don't send telemetry
+
 install:
   - git clone https://github.com/PowerShell/PowerShellEditorServices.git ../PowerShellEditorServices
   - ps: Install-Product node '6.9.2'