Update appveyor.yml

Author: ejsmith

appveyor.yml

@@ -16,7 +16,7 @@ assembly_info:
   file: Source\GlobalAssemblyInfo.cs
   assembly_version: "{version}.0"
   assembly_file_version: "{version}.0"
-  assembly_informational_version: "{version} $env:GIT_HASH"
+  assembly_informational_version: "{version} $(GIT_HASH)"
 
 before_build:
   - nuget restore -verbosity quiet