Use powershell syntax

JakeGinnivan · JakeGinnivan · commit b48a69cac429 · 2016-08-11T09:44:20.000+08:00
diff --git a/appveyor.yml b/appveyor.yml
@@ -8,7 +8,7 @@ configuration:
   - Debug
 
 build_script:
-  - ps: .\build.ps1 --githubToken={GitHubToken}
+  - ps: .\build.ps1 --githubToken=$env:GitHubToken
 
 test: off
 skip_tags: true
