Add AppVeyor environment vars to speed up CI builds

This change adds a couple of environment variables to speed up AppVeyor CI
builds:

- DOTNET_SKIP_FIRST_TIME_EXPERIENCE=true: Don't download unneeded
packages
- DOTNET_CLI_TELEMETRY_OPTOUT=true: Don't send telemetry

This information was gained from this blog post:
http://rehansaeed.com/cross-platform-devops-net-core/

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:
   - ps: |
       Install-PackageProvider -Name NuGet -MinimumVersion 2.8.5.201 -Force | Out-Null